bug#15898: Emacs aborts in buf_charpos_to_bytepos

[Barry OReilly]

Not having applied your patch yet, I reproduced the core dump in GDB and got a Lisp backtrace:

"line-end-position" (0xfffe9cc0)
"back-to-indentation" (0xfffe9de0)
"c-beginning-of-macro" (0xfffe9f70)
"c-extend-font-lock-region-for-macros" (0xfffea110)
"c-neutralize-syntax-in-and-mark-CPP" (0xfffea2c0)
0x1c01cb0 PVEC_COMPILED
"mapc" (0xfffea540)
"c-after-change" (0xfffea738)
"insert-file-contents" (0xffffad40)
"revert-buffer-insert-file-contents--default-function" (0xffffaec0)
"revert-buffer--default" (0xffffb028)
"revert-buffer" (0xffffb160)
"auto-revert-handler" (0xffffb300)
"auto-revert-buffers" (0xffffb558)
"apply" (0xffffb550)
"byte-code" (0xffffb660)
"timer-event-handler" (0xffffb928)

I'll apply your patch now.