(byte_combining_error): New function.

(byte_combining_error): New function.
(CHECK_BYTE_COMBINING_FOR_INSERT): Call byte_combining_error.
(insert_from_string_1): Set *(GPT_ADDR) to 0 before calling
CHECK_BYTE_COMBINING_FOR_INSERT.
(insert_from_buffer_1): Likewise.
(adjust_after_replace): Check the posibility of inhibitted byte
combining correctly.
(replace_range): Before signaling an error for inhibitted byte
combining, recover the buffer in a safe state.
(del_range_2): Check the posibility of inhibitted byte combining
correctly.