libsepol/cil: Fix out-of-bound read of file context pattern ending with "\"

Based on patch by Nicolas Iooss, who writes:
  OSS-Fuzz found a Heap-buffer-overflow in the CIL compiler when trying
  to compile the following policy:

    (sid SID)
    (sidorder(SID))
    (filecon "\" any ())
    (filecon "" any ())

  When cil_post_fc_fill_data() processes "\", it goes beyond the NUL
  terminator of the string. Fix this by returning when '\0' is read
  after a backslash.

To be consistent with the function compute_diffdata() in
refpolicy/support/fc_sort.py, also increment str_len in this case.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28484
Reported-by: Nicolas Iooss <nicolas.iooss@m4x.org>
Signed-off-by: James Carter <jwcart2@gmail.com>

libsepol/cil/src/cil_post.c

@@ -186,6 +186,13 @@ static void cil_post_fc_fill_data(struct fc_data *fc, const char *path)
 			break;
 		case '\\':
 			c++;
+			if (path[c] == '\0') {
+				if (!fc->meta) {
+					fc->stem_len++;
+				}
+				fc->str_len++;
+				return;
+			}
 			/* FALLTHRU */
 		default:
 			if (!fc->meta) {