Avoid calling memcpy() with a NULL source pointer and count == 0.
Avoid calling memcpy() with a NULL source pointer and count == 0.

As in commit 0a52d378b03b7d5a, avoid doing something that has undefined
results according to the C standard, even though in practice there does
not seem to be any problem with it.

This fixes two places in numeric.c that demonstrably could call memcpy()
with such arguments.  I looked through that file and didn't see any other
places with similar hazards; this is not to claim that there are not such
places in other files.

Per report from Piotr Stefaniak.  Back-patch to 9.5 which is where the
previous commit was added.  We're more or less setting a precedent that
we will not worry about this type of issue in pre-9.5 branches unless
someone demonstrates a problem in the field.

greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb