Improve diagnostics when an invalid reference is given to Get/Set*Field functions.

It may be worth treating NULL as a special case of "invalid" for an even
clearer diagnostic, but this is a huge leap forward from just crashing inside
CheckJNI:

    W( 4443) JNI ERROR: field operation on invalid reference ((nil))  (dalvikvm)
    E( 4443) VM aborting  (dalvikvm)

Change-Id: Ied7090a57becb10ca7c8cb4be56cd7c1f109add5
1 file changed