avoid returning NULL from xrealloc

This changeset is to kill future possibility of bugs similar to
CVE-2019-11932.   The vulnerability occurs when reallocarray(3)
(which is a variant of realloc(3) and roughly resembles our
ruby_xmalloc2()) returns NULL.  In our C API, ruby_xmalloc()
never returns NULL to raise NoMemoryError instead.  ruby_xfree()
does not return NULL by definition.  ruby_xrealloc() on the other
hand, _did_ return NULL, _and_ also raised sometimes.  It is very
confusing.  Let's not do that.  x-series APIs shall raise on
error and shall not return NULL.

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