vndk-def: Skip ndk ext check if no generic-refs.

After this commit, vndk_definition_tool.py will skip the NDK extension
check if generic references are not specified by the user.  This fixes
an unexpected AttributeError on NoneType object.

Test: Run ./vndk_definition_tool.py vndk command w/o --generic-refs ...
Change-Id: Ibcd960039c5acc4adabb1a2e2b5965a026e9f23a
1 file changed