review (S) for 6982537: Crash in Node*step_through_mergemem

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/6982537

6982537: Crash in Node*step_through_mergemem
Reviewed-by:

Code which was examining the type of memory was attempting to
introspect on the klass of TypeOopPtr but wasn't checking for NULL
which led to crashes when it looked at a primitive array type.  The
fix is to check for NULL.  I also updated similar code in escape.cpp
and corrected variable names to match the types.  Tested with failing
test from bug report.

src/share/vm/opto/memnode.cpp
src/share/vm/opto/escape.cpp