* doc.c: Integer and memory overflow fixes.

* doc.c: Integer and memory overflow fixes.

(get_doc_string_buffer_size): Now ptrdiff_t, not int.
(get_doc_string): Check for size calculation overflow.
Don't update size until allocation succeeds.
(get_doc_string, Fsubstitute_command_keys): Use ptrdiff_t, not
EMACS_INT, where ptrdiff_t will do.
(Fsubstitute_command_keys): Check for string overflow.