mem_alloc2(): New macro.  This is a variant of mem_alloc() that
returns the allocated size, which is equal to or greater than
the requested size.

mem_alloc_func(): Add the output parameter *size for the allocated size.
When it is set, adjust the parameter passed to mem_heap_alloc().

rec_copy_prefix_to_buf_old(), rec_copy_prefix_to_buf(): Use mem_alloc2()
instead of mem_alloc().