QueryFontMetrics nil pointer panic fix

[jackharrhy]

gographics/imagick@760c816 (v3.5.3, #309) added a GetLastError() check, but the C functions can return NULL without setting an error — e.g. when Freetype is not compiled in.

cdoubles := C.MagickQueryFontMetrics(mw.mw, dw.dw, cstext)
if err := mw.GetLastError(); err != nil { // NULL + no error → passes through
    return nil
}
defer relinquishMemory(unsafe.Pointer(cdoubles))
doubles := sizedDoubleArrayToFloat64Slice(cdoubles, 13) // <- panic: nil deref

Same issue in QueryMultilineFontMetrics.

Fix

Add an explicit nil check on cdoubles after the existing error check.

[justinfx]

Thank you for submitting a PR for your report! I'll get this merged and can tag it tomorrow

[justinfx]

Tagged v2.7.1 / v3.7.2