Fix a problem I introduced in r4208 when reducing the space used by

Fix a problem I introduced in r4208 when reducing the space used by
heap blocks.  The minimum size for redzones is now sizeof(void*), but
I forgot to ensure this.  Massif was asking for 0 byte redzones, and this
was screwing things up on 64-bit platforms, and Massif was dying very
quickly.  This should fix bugs #111090 and #111285.

The fact that Massif was this badly broken but there were only 2 bug reports
indicates that not many people are using it, at least not on AMD64.

I also added a regtest that does some basic malloc/realloc/free testing
for Massif, which would have caught this problem.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@4492