Issue 15777: Fix a refleak in _posixsubprocess.

It was exposed by 03c98d05b140 and dbbf3ccf72e8.

Misc/NEWS

@@ -109,6 +109,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #15777: Fix a refleak in _posixsubprocess.
+
 - Issue #15199: Fix JavaScript's default MIME type to application/javascript.
   Patch by Bohuslav Kabrda.
 

Modules/_posixsubprocess.c

@@ -566,8 +566,10 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     }
 
     exec_array = _PySequence_BytesToCharpArray(executable_list);
-    if (!exec_array)
+    if (!exec_array) {
+        Py_XDECREF(gc_module);
         return NULL;
+    }
 
     /* Convert args and env into appropriate arguments for exec() */
     /* These conversions are done in the parent process to avoid allocating