REGRESSION: incorrect null pointer check in discarded subexpression

The fix for 4449316 was overly aggressive. In fact, the JLS requires
no null pointer check when the subexpression is discarded.
It does, however, require evaluating the discarded subexpression.

The change for 4449316 should be modified to remove the null
pointer check in a discarded subexpression, but the subexpression
should still be evaluated fully. 4651185 should be closed as not a bug.
The fix for this bug needs no regression test because, as 4651185
documents, this area is adequately tested by JCK-compiler.