Guard against reallocation failure in pg_regress

Guard against reallocation failure in pg_regress

realloc() will return NULL on a failed reallocation, so the destination
pointer must be inspected to avoid null pointer dereference.  Further,
assigning the return value to the source pointer leak the allocation in
the case of reallocation failure.  Fix by using pg_realloc instead which
has full error handling.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/9FC7E603-9246-4C62-B466-A39CFAF454AE@yesql.se