schemagen: reject scalar root values when root schema is object/array

The generated RootBuilder crashed (undefined behavior on std::vector::back())
when a JSON document's root value was a scalar or null while the schema
declared a non-nullable object or array root. The stack starts empty for
object/array roots, but cbStringData, cbNumberData, and cbBool called
stack_.back() without checking for an empty stack.

Add an empty-stack guard to the three scalar callbacks so they reject
instead of crashing. cbNull already handles the empty-stack case.

Regression tests added for:
- non-nullable object root rejecting null, boolean, number, and string roots
- nullable object root rejecting scalar roots
- nullable array root rejecting scalar roots

Closes #16

contrib/schemagen/test_nullable_root.cpp

@@ -61,6 +61,18 @@ static void expectReject(nullable_object::RootBuilder &b, std::string in,
   }
 }
 
+static void expectReject(nullable_array::RootBuilder &b, std::string in,
+                         const char *what) {
+  WeaselJsonStatus s = parseStrided(b, in);
+  if (s == WeaselJson_REJECT) {
+    printf("ok   reject: %s\n", what);
+  } else {
+    printf("FAIL expected reject (%s) got status %d for: %s\n", what, s,
+           in.c_str());
+    ++failures;
+  }
+}
+
 int main() {
   // ---- nullable root object: valid document ----
   {
@@ -149,6 +161,34 @@ int main() {
     }
   }
 
+  // ---- nullable root object: scalar values rejected ----
+  {
+    nullable_object::RootBuilder b;
+    expectReject(b, "true", "boolean where nullable object expected (root)");
+  }
+  {
+    nullable_object::RootBuilder b;
+    expectReject(b, "123", "number where nullable object expected (root)");
+  }
+  {
+    nullable_object::RootBuilder b;
+    expectReject(b, R"("hi")", "string where nullable object expected (root)");
+  }
+
+  // ---- nullable root array: scalar values rejected ----
+  {
+    nullable_array::RootBuilder b;
+    expectReject(b, "true", "boolean where nullable array expected (root)");
+  }
+  {
+    nullable_array::RootBuilder b;
+    expectReject(b, "123", "number where nullable array expected (root)");
+  }
+  {
+    nullable_array::RootBuilder b;
+    expectReject(b, R"("hi")", "string where nullable array expected (root)");
+  }
+
   if (failures == 0) {
     printf("\nALL TESTS PASSED\n");
     return 0;