build: error properly if an invalid ndk is used

build: error properly if an invalid ndk is used

Currently, if buildjni is not used but ndk is set to an invalid value,
the build would start but $ANDROID_NDK would be empty. This is happening
in VLC, which results in very confusing errors.

If a build uses a ndk= value that is not set up, such as r11/r12 which
we do not have yet, it should error with "NDK version could not be
found". It does with this change.