gbm: fix potential NULL deref of mapImage/unmapImage.
gbm: fix potential NULL deref of mapImage/unmapImage.

The mapImage/unmapImage functions of DRIimage extension can be NULL,
so we should add additional check for them.

Cc: <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Chuanbo Weng <4145c40a11b367664079fb2725d5e44cc6d84d1d@intel.com>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler