Fix subtle memory corruption bug

Reported by William Ma on the link-grammar mailing list.
Problem when using link-grammar with opencog.

Found it.  Apparently, /usr/lib/libbfd-2.24-system.so has a symbol
called "xrealloc" in it, and opencog links to libbfd to perform pretty
stack-trace printing. Due to dynamic linking order, the xrealloc in
link-grammar gets resolved by the symbol in libbfd, instead of the one
in liblink-grammar.  The libbfd version is expecting the new size as the
second argument. But we pass the old size in that location. So the
returned arrays are always too small, and memory corruption results.

The solution is to stop using xrealloc, which no longer serves a purpose
anyway.
Remove usage of xrealloc, per previous commit msg
Merge pull request #47 from linas/mem

Fix crazy libbfd symbol collision