make WeaselJson_OVERFLOW a terminal state like WeaselJson_REJECT

When a parse step returned WeaselJson_OVERFLOW, the pushdown stack was
left corrupted (frames had been popped before the failing push), but the
overflow status was not made sticky the way WeaselJson_REJECT is. A
subsequent end-of-data call WeaselJsonParser_parse(parser, nullptr, 0)
could then dispatch through the corrupted stack and return WeaselJson_OK,
accepting an incomplete, invalid too-deeply-nested document as valid JSON.

Add an `overflowed` flag, set it whenever a continuation returns
WeaselJson_OVERFLOW, and short-circuit parse() to return
WeaselJson_OVERFLOW on every subsequent call. reset() clears the flag so a
reused parser can accept a different document. This mirrors the existing
stickiness handling for WeaselJson_REJECT.

Closes #52

src/parser3.h

@@ -139,7 +139,8 @@ struct Parser3 {
     stackPtr = stack();
     std::ignore = push({N_VALUE, N_WHITESPACE, T_EOF});
     inKey = false;
-    terminalStatus = WeaselJson_OK;
+    rejected = false;
+    overflowed = false;
     utf8Codepoint = 0;
     utf16Surrogate = 0;
     minCodepoint = 0;
@@ -162,7 +163,8 @@ struct Parser3 {
   NumDfa numDfa;
   Utf8Dfa strDfa;
   bool inKey = false;
-  WeaselJsonStatus terminalStatus = WeaselJson_OK;
+  bool rejected = false;
+  bool overflowed = false;
 
 #ifndef HAS_MUSTTAIL
   char *stashBufForTrampoline;
@@ -1070,12 +1072,16 @@ constexpr inline struct ContinuationTable {
 inline WeaselJsonStatus Parser3::parse(char *buf, int len) {
   this->dataBegin = this->writeBuf = buf;
 
-  if (this->terminalStatus != WeaselJson_OK) [[unlikely]] {
-    return this->terminalStatus;
+  if (this->rejected) [[unlikely]] {
+    return WeaselJson_REJECT;
+  }
+
+  if (this->overflowed) [[unlikely]] {
+    return WeaselJson_OVERFLOW;
   }
 
   if (len < 0) [[unlikely]] {
-    this->terminalStatus = WeaselJson_REJECT;
+    this->rejected = true;
     return WeaselJson_REJECT;
   }
 
@@ -1085,8 +1091,10 @@ inline WeaselJsonStatus Parser3::parse(char *buf, int len) {
   // range.
   ContinuationStatus status =
       symbolTables.continuations[top()](this, buf, buf + len);
-  if (status == WeaselJson_REJECT || status == WeaselJson_OVERFLOW) {
-    this->terminalStatus = WeaselJsonStatus(status);
+  if (status == WeaselJson_REJECT) {
+    this->rejected = true;
+  } else if (status == WeaselJson_OVERFLOW) {
+    this->overflowed = true;
   }
   return WeaselJsonStatus(status);
 #else
@@ -1095,8 +1103,10 @@ inline WeaselJsonStatus Parser3::parse(char *buf, int len) {
   while ((result = symbolTables.continuations[top()](
               this, stashBufForTrampoline, buf + len)) == kBounce)
     ;
-  if (result == WeaselJson_REJECT || result == WeaselJson_OVERFLOW) {
-    this->terminalStatus = WeaselJsonStatus(result);
+  if (result == WeaselJson_REJECT) {
+    this->rejected = true;
+  } else if (result == WeaselJson_OVERFLOW) {
+    this->overflowed = true;
   }
   return WeaselJsonStatus(result);
 #endif