gdi32/tests: make sure to use return value (LLVM/Clang)

[Austin English]

Ran WTB first, since fonts/locales are often tricky:
https://testbot.winehq.org/JobDetails.pl?Key=8890

-- 
-Austin
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diff --git a/dlls/gdi32/tests/font.c b/dlls/gdi32/tests/font.c
index 3ea883a..8bce92b 100644
--- a/dlls/gdi32/tests/font.c
+++ b/dlls/gdi32/tests/font.c
@@ -1136,6 +1136,7 @@ static void test_GetGlyphIndices(void)
     lf.lfCharSet = ANSI_CHARSET;
 
     hfont = CreateFontIndirectA(&lf);
+    ok(hfont != 0, "CreateFontIndirectEx failed\n");
     ok(GetTextMetrics(hdc, &textm), "GetTextMetric failed\n");
     if (textm.tmCharSet == ANSI_CHARSET)
     {