xmlparse.c: Fix dangling pointer caused by use of realloc

xmlparse.c: Fix dangling pointer caused by use of realloc

Variables pool->ptr and pool->start point to addresses
that may have been freed if realloc chose the path of
a new base address.  So we do the math on these pointers
while they are not dangling, yet.

For a related article:
http://trust-in-soft.com/dangling-pointer-indeterminate/