libnativebridge: fix a potential null deref

Caught by the static analyzer:
> art/libnativebridge/native_bridge.cc:239:40: warning: Access to field
'version' results in a dereference of a null pointer (loaded from
variable 'callbacks') [clang-analyzer-core.NullDereference]

Bug: None
Test: TreeHugger
Change-Id: Ia0eb0daac6ee3b07b9ffd615cec9f0a431de2900
1 file changed