gatt: fix potential use of NULL

It seems that `jniThrowIOException` doesn't unwind the stack -- it just
stores some info so that when the _caller_ returns control to the JVM,
the JVM can handle unwinding of Java frames. Hence, we should be
returning after these.

Caught by the static analyzer:
> packages/apps/Bluetooth/jni/com_android_bluetooth_gatt.cpp:1536:23:
warning: Array access (from variable 'irkBytes') results in a null
pointer dereference [clang-analyzer-core.NullDereference]

Bug: 194432570
Test: TreeHugger
Tag: #feature
Change-Id: I11a690b10de825dfb855685d68aafac5b6273ca4