ART: JNI ExceptionDescribe crashes if no exception occurred

Some tests are calling ExceptionDescribe without checking if
we have an exception occurred. The most JVM's like Dalvik can
handle this in a good way, but art crashes with JNI error.

This adds a check in art::ExceptionDescribe for a case when it
called without exception.

Change-Id: Id9eddcc73e78b1197109be5a6340f9ff60940c74
Signed-off-by: Alexei Zavjalov <alexei.zavjalov@intel.com>