shell32/tests: make sure return value is used (LLVM/Clang)

[Austin English]

-- 
-Austin
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diff --git a/dlls/shell32/tests/shlexec.c b/dlls/shell32/tests/shlexec.c
index 252aed7..2933740 100644
--- a/dlls/shell32/tests/shlexec.c
+++ b/dlls/shell32/tests/shlexec.c
@@ -343,6 +343,7 @@ static void create_test_verb_dde(const char* extension, const char* verb,
     if (rawcmd)
     {
         rc=RegSetValueEx(hkey_cmd, NULL, 0, REG_SZ, (LPBYTE)cmdtail, strlen(cmdtail)+1);
+        assert(rc==ERROR_SUCCESS);
     }
     else
     {