schemagen: handle WeaselJsonParser_create failure in RootBuilder

If WeaselJsonParser_create returns nullptr (e.g. negative stack size or allocation failure), set the existing error_ flag so that subsequent feed()/finish() calls return WeaselJson_REJECT instead of dereferencing the null parser_.

Also add a regression test in test_gen.cpp that constructs a RootBuilder with an invalid stack size and verifies it rejects without crashing.

Closes #36

contrib/schemagen/test_gen.cpp

@@ -68,6 +68,15 @@ int main() {
     expectReject(json, "unknown key in strict root");
   }
 
+  // ---- invalid stack size is rejected without crashing ----
+  {
+    RootBuilder b(-1);
+    char buf[] = "null";
+    WeaselJsonStatus s = b.feed(buf, sizeof(buf) - 1);
+    CHECK(s == WeaselJson_REJECT);
+    printf("ok   invalid stack size rejected, not crashed\n");
+  }
+
   {
     std::string json = R"({
       "name": "Ada É",

contrib/schemagen/weaseljson_schemagen.py

@@ -892,6 +892,10 @@ public:
   explicit RootBuilder(int stackSize = 1024) {{
     cb_ = makeCallbacks();
     parser_ = WeaselJsonParser_create(stackSize, &cb_, this, 0);
+    if (!parser_) {{
+      error_ = true;
+      return;
+    }}
 {self._ctor_body()}
   }}
   ~RootBuilder() {{ if (parser_) WeaselJsonParser_destroy(parser_); }}