Fix CVE-2016-6265 (use after free in pdf_load_xref()).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-6265
https://security-tracker.debian.org/tracker/CVE-2016-6265

Patch copied from upstream source repository:

http://git.ghostscript.com/?p=mupdf.git;h=fa1936405b6a84e5c9bb440912c23d532772f958

diff --git a/source/pdf/pdf-xref.c b/source/pdf/pdf-xref.c
index 576c315..3222599 100644
--- a/source/pdf/pdf-xref.c
+++ b/source/pdf/pdf-xref.c
@@ -1184,8 +1184,14 @@ pdf_load_xref(fz_context *ctx, pdf_document *doc, pdf_lexbuf *buf)
 				fz_throw(ctx, FZ_ERROR_GENERIC, "object offset out of range: %d (%d 0 R)", (int)entry->ofs, i);
 		}
 		if (entry->type == 'o')
-			if (entry->ofs <= 0 || entry->ofs >= xref_len || pdf_get_xref_entry(ctx, doc, entry->ofs)->type != 'n')
-				fz_throw(ctx, FZ_ERROR_GENERIC, "invalid reference to an objstm that does not exist: %d (%d 0 R)", (int)entry->ofs, i);
+		{
+			/* Read this into a local variable here, because pdf_get_xref_entry
+			 * may solidify the xref, hence invalidating "entry", meaning we
+			 * need a stashed value for the throw. */
+			fz_off_t ofs = entry->ofs;
+			if (ofs <= 0 || ofs >= xref_len || pdf_get_xref_entry(ctx, doc, ofs)->type != 'n')
+				fz_throw(ctx, FZ_ERROR_GENERIC, "invalid reference to an objstm that does not exist: %d (%d 0 R)", (int)ofs, i);
+		}
 	}
 }
 
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<div class='path'>path: <a href='/guix/log/?id=1f09284034ba256246d9a02291a46982c34ccf83'>root</a>/<a href='/guix/log/build-aux?id=1f09284034ba256246d9a02291a46982c34ccf83'>build-aux</a>/<a href='/guix/log/build-aux/test-driver.scm?id=1f09284034ba256246d9a02291a46982c34ccf83'>test-driver.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/build-aux/test-driver.scm?id=1f09284034ba256246d9a02291a46982c34ccf83&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-03-22 15:02:33 +0100'>2017-03-22</span></td><td><a href='/guix/commit/build-aux/test-driver.scm?id=73a46451af333c77d2e79aa8764f51be8c34d1ae'>build: 'test-driver.scm' prints Scheme values with 'write'.</a><span class='msg-avail'>...<span class='msg-tooltip'>Previously it would print them with 'display', which was inconvenient.

* build-aux/test-driver.scm (test-display): Use ~S for VALUE.
(test-runner-gnu): Use 'format' instead of 'test-display' for the test
name, location, and result kind.
</span></span></td><td>Ludovic Courtès</td></tr>