userenv/tests: Fix a memory leak (valgrind)

[Bruno Jesus]

For bug https://bugs.winehq.org/show_bug.cgi?id=36588
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diff --git a/dlls/userenv/tests/userenv.c b/dlls/userenv/tests/userenv.c
index b085d17..9e4179f 100644
--- a/dlls/userenv/tests/userenv.c
+++ b/dlls/userenv/tests/userenv.c
@@ -154,6 +154,7 @@ static void test_create_env(void)
         r = get_env(env[i], "SystemRoot", &st);
         ok(!strcmp(st, "SystemRoot=overwrite"), "%s\n", st);
         expect(TRUE, r);
+        if (r) HeapFree(GetProcessHeap(), 0, st);
     }
 
     /* Test for common environment variables (NT4 and higher) */