zink: Check fopen result.
Fix warning reported by Coverity.
Dereference null return value (NULL_RETURNS)
dereference: Dereferencing a pointer that might be NULL fp when calling
fwrite.
Fixes: 8d46e35d16e3 ("zink: introduce opengl over vulkan")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
cc: @kusma