Fix bug in backwards_compatibility test case.

The memset() filled also the extensions field, which was just
waiting for a crash to happen.

tests/backwards_compatibility/decode_legacy.c

@@ -22,10 +22,7 @@
    the decoding and checks the fields. */
 bool check_alltypes(pb_istream_t *stream, int mode)
 {
-    AllTypes alltypes;
-    
-    /* Fill with garbage to better detect initialization errors */
-    memset(&alltypes, 0xAA, sizeof(alltypes));
+    AllTypes alltypes = {0};
     
     if (!pb_decode(stream, AllTypes_fields, &alltypes))
         return false;