java/jni: NewStringUTF

Signed-off-by: Pekka Enberg <penberg@iki.fi>
java/jni: Fix API stubs to abort()

Use proper API stub that aborts the process rather than setting the JNI
functions to nullptr and SIGSEGV'ing in mysterious ways.

Signed-off-by: Pekka Enberg <penberg@iki.fi>