Ignore invalid base64 messages

When base64 encoding is enabled, ignore any messages that fail base64
decoding.

Previously signed-unsigned integer conversion would cause the return
value of b64_pton() on error (a negative integer) to be converted to a
large value. The attempt to allocate this value would force xmppipe to
exit.

src/xmppipe.c

@@ -966,7 +966,12 @@ handle_message(xmpp_conn_t * const conn, xmpp_stanza_t * const stanza,
         /* Does not need to be NULL terminated, buf is passed with length */
         size_t len = strlen(message) * 3 / 4;
         char *buf = xmppipe_calloc(len, 1);
-        size_t n = b64_pton(message, (u_char *)buf, len);
+        int n = b64_pton(message, (u_char *)buf, len);
+        if (n <= 0 || n > len) {
+            /* Not a base64 message */
+            free(buf);
+            return 1;
+        }
         emessage = xmppipe_nfmt(buf,n);
         free(buf);
     }