pythongh-105375: Improve PyErr_WarnExplicit() error handling (python#105610)

Bail on first error to prevent exceptions from possibly being
overwritten.

Author: erlend-aasland

Misc/NEWS.d/next/C API/2023-06-09-23-34-25.gh-issue-105375.n7amiF.rst

@@ -0,0 +1,2 @@
+Fix a bug in :c:func:`PyErr_WarnExplicit` where an exception could end up
+being overwritten if the API failed internally.

Python/_warnings.c

@@ -1298,25 +1298,29 @@ PyErr_WarnExplicit(PyObject *category, const char *text,
                    const char *module_str, PyObject *registry)
 {
     PyObject *message = PyUnicode_FromString(text);
+    if (message == NULL) {
+        return -1;
+    }
     PyObject *filename = PyUnicode_DecodeFSDefault(filename_str);
+    if (filename == NULL) {
+        Py_DECREF(message);
+        return -1;
+    }
     PyObject *module = NULL;
-    int ret = -1;
-
-    if (message == NULL || filename == NULL)
-        goto exit;
     if (module_str != NULL) {
         module = PyUnicode_FromString(module_str);
-        if (module == NULL)
-            goto exit;
+        if (module == NULL) {
+            Py_DECREF(filename);
+            Py_DECREF(message);
+            return -1;
+        }
     }
 
-    ret = PyErr_WarnExplicitObject(category, message, filename, lineno,
-                                   module, registry);
-
- exit:
-    Py_XDECREF(message);
+    int ret = PyErr_WarnExplicitObject(category, message, filename, lineno,
+                                       module, registry);
     Py_XDECREF(module);
-    Py_XDECREF(filename);
+    Py_DECREF(filename);
+    Py_DECREF(message);
     return ret;
 }