libsepol: allocate enough space to hold filename in trans rules

There is an off by one bug in which the filename length stored with
filename_trans_rules is stored as strlen (aka, no nul) however the
code to allocate space and read the name back in from policy only
allocates len, and not the len + 1 needed to hold the nul.  Allocate
enough space for the nul.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Dan Walsh <dwalsh@redhat.com>

libsepol/src/policydb.c

@@ -2380,7 +2380,7 @@ int filename_trans_read(filename_trans_t **t, struct policy_file *fp)
 			return -1;
 		len = le32_to_cpu(buf[0]);
 
-		name = calloc(len, sizeof(*name));
+		name = calloc(len + 1, sizeof(*name));
 		if (!name)
 			return -1;