Fix 383275 - massif valgrind: m_xarray.c:162 (ensureSpaceXA): Assertion '!xa->arr...

Fix 383275 - massif valgrind: m_xarray.c:162 (ensureSpaceXA): Assertion '!xa->arr' failed

When a massif xtree snapshot is taken when no allocation was done,
the xtree contains no exe context.
The data structure ips_order_xecu is then szied to 0 using VG_(hintSizeXA).
m_xarray.c then allocates an empty array, while later on, a zero size
is expected to correspond to no allocated array.

Fix the problem in m_xarray.c, by not doing any allocation if the
size hint is 0.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16469