Fix bug #18995 with compiling w32heap.c with -funroll-loops.

 src/w32heap.c (allocate_heap): Set the lower limit of heap at 8MB.
Fix bug #18982 with assertion violations when reverting buffers.

 src/fileio.c (Finsert_file_contents): Invalidate buffer caches also
 when the inserted text does not need decoding.