unhook_system_services: bail on systab == NULL
unhook_system_services: bail on systab == NULL

Prevent unhook_system_services() from dereferencing a NULL systab, which
may occur if hook_system_services() has never been called.

Signed-off-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>

rhinstaller/shim,android-ia/platform_external_uefi_shim,CyanogenMod/android_external_uefi_shim,CyanogenMod/android_external_uefi_shim,rhinstaller/shim,android-ia/platform_external_uefi_shim,rhinstaller/shim,android-ia/platform_external_uefi_shim,CyanogenMod/android_external_uefi_shim