mkfs.jffs2: fix integer underflow in jffs2_rtime_compress()

mkfs.jffs2: fix integer underflow in jffs2_rtime_compress()

When '*dstlen' is 0 or 1, comparison will return wrong result.  Reported
by valgrind as

==5919== Invalid write of size 1
==5919==    at 0x40564E: jffs2_rtime_compress (compr_rtime.c:51)
==5919==    by 0x40676B: jffs2_compress (compr.c:246)
==5919==    by 0x403EE4: recursive_populate_directory (mkfs.jffs2.c:884)
==5919==  Address 0x4e1bdb1 is 0 bytes after a block of size 1 alloc'd
==5919==    at 0x4A0515D: malloc (vg_replace_malloc.c:195)
==5919==    by 0x40671C: jffs2_compress (compr.c:229)
==5919==    by 0x403EE4: recursive_populate_directory (mkfs.jffs2.c:884)

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>