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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<div class='path'>path: <a href='/guix/log/?id=8d428985723dab0e203b965545fb708709a28882'>root</a>/<a href='/guix/log/tests?id=8d428985723dab0e203b965545fb708709a28882'>tests</a>/<a href='/guix/log/tests/guix-environment.sh?id=8d428985723dab0e203b965545fb708709a28882'>guix-environment.sh</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/tests/guix-environment.sh?id=8d428985723dab0e203b965545fb708709a28882&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-10-31 14:49:47 +0200'>2021-10-31</span></td><td><a href='/guix/commit/tests/guix-environment.sh?id=bc5155b952ae8bdbc56aded4d8d39768b4e2a7d4'>Merge remote-tracking branch 'origin/master' into core-updates-frozen</a></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2021-10-25 19:02:32 +0200'>2021-10-25</span></td><td><a href='/guix/commit/tests/guix-environment.sh?id=10208952eaf0834b9cdf341bc75463ed033af315'>environment: Add tests for '--profile'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to a643deac2de81755a1843a3b41dd53857678bebc.

* tests/guix-environment-container.sh, tests/guix-environment.sh: Add
tests for '--profile'.
</span></span></td><td>Ludovic Courtès</td></tr>