Clear entire "location" in value constructor

Clear entire "location" in value constructor

My recent change to allocate values with "new" may have introduced a
small bug.  In particular, the previous code allocated with XCNEW, but
the new code only clears a part of the "location" field in the
constructor.  I didn't try very hard to actually trigger a bug here,
the problem remains theoretical.

This patch just arranges to initialize the entire location.

Regression tested by the buildbot.

2018-05-25  Tom Tromey  <tom@tromey.com>

	* value.c (value::location): Initialize.