Fix build against libyaml 0.2.5.

Taken from upstream:
https://github.com/tlsa/libcyaml/commit/0e947264b947f51f7ea785068637c4bfce5d4171

diff --git a/test/units/save.c b/test/units/save.c
--- a/test/units/save.c
+++ b/test/units/save.c
@@ -3476,7 +3476,7 @@ static bool test_save_sequence_null_values_int(
 		ttest_report_ctx_t *report,
 		const cyaml_config_t *config)
 {
-	static const unsigned char ref[] =
+	static const unsigned char ref1[] =
 		"---\n"
 		"- 7\n"
 		"- 6\n"
@@ -3487,6 +3487,18 @@ static bool test_save_sequence_null_values_int(
 		"- \n"
 		"- 0\n"
 		"...\n";
+	/* As of libyaml 0.2.5, trailing spaces are not emitted. */
+	static const unsigned char ref2[] =
+		"---\n"
+		"- 7\n"
+		"- 6\n"
+		"- 5\n"
+		"-\n"
+		"- 3\n"
+		"- 2\n"
+		"-\n"
+		"- 0\n"
+		"...\n";
 	static const int d[] = { 7, 6, 5, 4, 3, 2, 1, 0 };
 	static const int *data[] = { d + 0, d + 1, d + 2, NULL,
 	                             d + 4, d + 5, NULL, d + 7, };
@@ -3515,11 +3527,12 @@ static bool test_save_sequence_null_values_int(
 		return ttest_fail(&tc, cyaml_strerror(err));
 	}
 
-	if (len != YAML_LEN(ref) || memcmp(ref, buffer, len) != 0) {
+	if ((len != YAML_LEN(ref1) || memcmp(ref1, buffer, len) != 0) &&
+	    (len != YAML_LEN(ref2) || memcmp(ref2, buffer, len) != 0)) {
 		return ttest_fail(&tc, "Bad data:\n"
 				"EXPECTED (%zu):\n\n%.*s\n\n"
 				"GOT (%zu):\n\n%.*s\n",
-				YAML_LEN(ref), YAML_LEN(ref), ref,
+				YAML_LEN(ref1), YAML_LEN(ref1), ref1,
 				len, len, buffer);
 	}
 
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<div class='path'>path: <a href='/guix/log/?id=ea042113585d09bb9c12598f287a5a586a93a8cf'>root</a>/<a href='/guix/log/gnu?id=ea042113585d09bb9c12598f287a5a586a93a8cf'>gnu</a>/<a href='/guix/log/gnu/artwork.scm?id=ea042113585d09bb9c12598f287a5a586a93a8cf'>artwork.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/artwork.scm?id=ea042113585d09bb9c12598f287a5a586a93a8cf&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-06-10 23:11:07 +0200'>2019-06-10</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=226a3605d6cabeb71247a732a99d625c9cfafa74'>artwork: Update snapshot to 2f2fe74.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/artwork.scm (%artwork-repository): Update to 2f2fe74.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Diego Nicola Barbato</td></tr>
<tr><td><span title='2019-03-05 23:08:53 +0100'>2019-03-05</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=5387971a9ecf511e1dbbac992af3808437e58d56'>artwork: Update snapshot to b975164.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/artwork.scm (%artwork-repository): Update to b975164.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-02-10 23:25:01 +0100'>2019-02-10</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=5e6a5aeebd7b735e5d5f2eed496cc1c7e8a5b4c9'>artwork: Update snapshot to e951905.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/artwork.scm (%artwork-repository): Update to e951905.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-05 00:08:47 +0100'>2018-11-05</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=688d1a01e45eb1277a6337987bfd53f3fe3caaec'>artwork: Clone over HTTPS.</a><span class='msg-avail'>...<span class='msg-tooltip'>Suggested by Leo Famulari &lt;leo@famulari.name&gt;
at &lt;https://lists.gnu.org/archive/html/help-guix/2018-11/msg00039.html&gt;.

* gnu/artwork.scm (%artwork-repository): Switch to HTTPS URL.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-06-07 15:58:16 -0400'>2017-06-07</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=94db92f78ec9c849f9c8da05d4d35356909f9fbe'>artwork: Use a descriptive name for the source directory.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/artwork.scm (%artwork-repository): Set a descriptive file-name and
use the full commit hash when fetching.
</span></span></td><td>Leo Famulari</td></tr>