Fix a bug whereby 'mformat' could end up passing uninitialized bytes
to write(2).  This could be reproduced with:

  mformat -C -f 1440 -L 16  -N 77777777 -i  /tmp/x ::

where the output of /tmp/x would be non-deterministic.

Patch by Ludovic Courtès <ludo@gnu.org>.

--- mtools-4.0.23/mformat.c	2019-04-21 00:12:01.496116195 +0200
+++ mtools-4.0.23/mformat.c	2019-04-21 00:12:36.675967157 +0200
@@ -927,6 +927,7 @@ void mformat(int argc, char **argv, int
 	
 	char *endptr;
 
+	memset(&boot.bytes, '\0', sizeof boot);
 	hs = hs_set = 0;
 	argtracks = 0;
 	argheads = 0;

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<tr><td><span title='2019-01-21 09:59:41 +0100'>2019-01-21</span></td><td><a href='/guix/commit/TODO?id=b93d2602f72dbbd9334a4f5c75b8721e1f2fa35a'>Update 'TODO'</a><span class='msg-avail'>...<span class='msg-tooltip'>* TODO (Installer): Add Pierre Neidhardt suggestions described here:
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</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:11:15 +0100'>2019-01-17</span></td><td><a href='/guix/commit/TODO?id=11b68c90cd89288906c595d5c31a0413ed8a496f'>Update 'TODO'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* TODO (Installer): New section.
</span></span></td><td>Mathieu Othacehe</td></tr>