libselinux/utils: drop reachable assert in sefcontext_compile

The two asserts following qsort(3) where useful during development to
ensure the comparison function and the corresponding pointer handling
were correct.  They however do not take into account an empty file
context definition file containing no definitions and thus `stab->nel`
being NULL.  Drop the two asserts.

Also return early to not depend on whether calloc(3) called with a size
of zero returns NULL or a special value.

Reported-by: Petr Lautrbach <lautrbach@redhat.com>
Closes: https://lore.kernel.org/selinux/87jzchqck5.fsf@redhat.com/
Fixes: 92306da ("libselinux: rework selabel_file(5) database")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Tested-by: Petr Lautrbach <lautrbach@redhat.com>
Acked-by: James Carter <jwcart2@gmail.com>

Author: cgzones

libselinux/utils/sefcontext_compile.c

@@ -188,6 +188,9 @@ static int write_sidtab(FILE *bin_file, const struct sidtab *stab)
 	if (len != 1)
 		return -1;
 
+	if (stab->nel == 0)
+		return 0;
+
 	/* sort entries by id */
 	sids = calloc(stab->nel, sizeof(*sids));
 	if (!sids)
@@ -203,8 +206,6 @@ static int write_sidtab(FILE *bin_file, const struct sidtab *stab)
 	}
 	assert(index == stab->nel);
 	qsort(sids, stab->nel, sizeof(struct security_id), security_id_compare);
-	assert(sids[0].id == 1);
-	assert(sids[stab->nel - 1].id == stab->nel);
 
 	/* write raw contexts sorted by id */
 	for (uint32_t i = 0; i < stab->nel; i++) {