EFI: Populate descriptor_count value only when image_info_size is not zero

The GetImageInfo function of the Firmware Mangement Protocol(FMP) gets
called initially to query the size of the image descriptor array that
would have to be allocated. During this call, the rest of the function
arguments, specifically pointers might be passed as NULL. Do not
populate the descriptor_count value before it is known that the call
to GetImageInfo has been made with the allocated buffer for the image
descriptors.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>