Avoid automatic variables in functions using setjmp.

According to https://en.cppreference.com/w/c/program/setjmp automatic variables
modified in a function calling setjmp can have indeterminate values.  Instead,
refactor all functions calling setjmp so that the function calling setjmp
doesn’t have any local variables.

Part I: JSON encoder/decoder.  The code was previously in compliance, but a
convention of avoiding non-const local variables in functions calling setjmp
will make the compliance more obvious.

PiperOrigin-RevId: 502927863

upb/json/decode.c

@@ -1446,6 +1446,14 @@ static void jsondec_wellknown(jsondec* d, upb_Message* msg,
   }
 }
 
+static bool upb_JsonDecoder_Decode(jsondec* const d, upb_Message* const msg,
+                                   const upb_MessageDef* const m) {
+  if (UPB_SETJMP(d->err)) return false;
+
+  jsondec_tomsg(d, msg, m);
+  return true;
+}
+
 bool upb_JsonDecode(const char* buf, size_t size, upb_Message* msg,
                     const upb_MessageDef* m, const upb_DefPool* symtab,
                     int options, upb_Arena* arena, upb_Status* status) {
@@ -1465,8 +1473,5 @@ bool upb_JsonDecode(const char* buf, size_t size, upb_Message* msg,
   d.debug_field = NULL;
   d.is_first = false;
 
-  if (UPB_SETJMP(d.err)) return false;
-
-  jsondec_tomsg(&d, msg, m);
-  return true;
+  return upb_JsonDecoder_Decode(&d, msg, m);
 }

upb/json/encode.c

@@ -775,6 +775,17 @@ static size_t jsonenc_nullz(jsonenc* e, size_t size) {
   return ret;
 }
 
+static size_t upb_JsonEncoder_Encode(jsonenc* const e,
+                                     const upb_Message* const msg,
+                                     const upb_MessageDef* const m,
+                                     const size_t size) {
+  if (UPB_SETJMP(e->err) != 0) return -1;
+
+  jsonenc_msgfield(e, msg, m);
+  if (e->arena) upb_Arena_Free(e->arena);
+  return jsonenc_nullz(e, size);
+}
+
 size_t upb_JsonEncode(const upb_Message* msg, const upb_MessageDef* m,
                       const upb_DefPool* ext_pool, int options, char* buf,
                       size_t size, upb_Status* status) {
@@ -789,9 +800,5 @@ size_t upb_JsonEncode(const upb_Message* msg, const upb_MessageDef* m,
   e.status = status;
   e.arena = NULL;
 
-  if (UPB_SETJMP(e.err)) return -1;
-
-  jsonenc_msgfield(&e, msg, m);
-  if (e.arena) upb_Arena_Free(e.arena);
-  return jsonenc_nullz(&e, size);
+  return upb_JsonEncoder_Encode(&e, msg, m, size);
 }