Fix bug #17622 with crashes in mmap routines.

Fix bug #17622 with crashes in mmap routines.

 src/buffer.c (init_buffer): Accept an argument 'initialized'.
 [USE_MMAP_FOR_BUFFERS]: If 'initialized' is non-zero, reset
 mmap_regions and mmap_fd, to avoid referencing stale data from the
 dump phase.  Add an assertion for buffer text of buffers created
 in temacs before this function is called.
 (mmap_regions_1, mmap_fd_1): Remove unused variables.
 src/lisp.h (init_buffer): Update prototype.
 src/emacs.c (main): Pass 'initialized' as the argument to init_buffer.