check WeaselJsonParser_create return value in Python bindings

Raise ValueError from __init__ when the C constructor returns NULL,
instead of storing a NULL pointer that segfaults on parse()/reset().
Add defensive RuntimeError checks in parse() and reset() for closed
or failed parsers.

Add a test covering stackSize values that the C API rejects (-1, 0, 1, 2).

Fixes #23

test_python_bindings.py

@@ -84,7 +84,19 @@ def test_mixed_values():
     assert recorder.events.count("null") == 1
 
 
+def test_create_rejects_too_small_stack():
+    for stack_size in (-1, 0, 1, 2):
+        try:
+            parser = weaseljson.WeaselJsonParser(Recorder(), stackSize=stack_size)
+        except ValueError:
+            continue
+        # If creation unexpectedly succeeds, close it cleanly and fail the test.
+        parser.close()
+        raise AssertionError(f"expected ValueError for stackSize={stack_size}")
+
+
 if __name__ == "__main__":
     test_object_keys_routed_correctly()
     test_mixed_values()
+    test_create_rejects_too_small_stack()
     print("python bindings ok")

weaseljson.py

@@ -117,12 +117,24 @@ class WeaselJsonParser:
             self.voidp_callbacks,
             0,
         )
+        if self.p is None:
+            raise ValueError(
+                "WeaselJsonParser_create returned NULL; "
+                "check stackSize (must be positive and large enough) "
+                "and available memory"
+            )
+
+    def _check_open(self):
+        if self.p is None:
+            raise RuntimeError("parser has been closed or creation failed")
 
     def parse(self, data: bytes) -> WeaselJsonStatus:
+        self._check_open()
         buf = (ctypes.c_ubyte * len(data)).from_buffer(bytearray(data))
         return self._lib.WeaselJsonParser_parse(self.p, buf, len(data))
 
     def reset(self):
+        self._check_open()
         self._lib.WeaselJsonParser_reset(self.p)
 
     def __enter__(self):