Fixed some c89 errors.

upb/decode.c

@@ -224,8 +224,8 @@ static upb_array *upb_getorcreatearr(upb_decframe *frame,
 
 static void upb_sethasbit(upb_decframe *frame,
                           const upb_msglayout_field *field) {
-  UPB_ASSERT(field->presence > 0);
   int32_t hasbit = field->presence;
+  UPB_ASSERT(field->presence > 0);
   frame->msg[hasbit / 8] |= (1 << (hasbit % 8));
 }
 

upb/encode.c

@@ -134,8 +134,8 @@ static uint32_t upb_readcase(const char *msg, const upb_msglayout_field *f) {
 }
 
 static bool upb_readhasbit(const char *msg, const upb_msglayout_field *f) {
-  UPB_ASSERT(f->presence > 0);
   uint32_t hasbit = f->presence;
+  UPB_ASSERT(f->presence > 0);
   return msg[hasbit / 8] & (1 << (hasbit % 8));
 }