gdiplus: Check for null in GdipGetFontHeight

[Vincent Povirk]

This fails the gdiplus:font tests.

font.c:550: Test failed: Expected 0, got 2
font.c:627: Test failed: font_height
63875314620738696996458594304.000000 != 15.691406
font.c:640: Test failed: font_height
63875314620738696996458594304.000000 != 15.691406
font.c:550: Test failed: Expected 0, got 2
font.c:668: Test failed: font_height
63875314620738696996458594304.000000 != 13.277344
font.c:681: Test failed: font_height
63875314620738696996458594304.000000 != 13.277344
font.c:550: Test failed: Expected 0, got 2
font.c:710: Test failed: font_height
63875314620738696996458594304.000000 != 15.691406