Fix a segfault / internal compiler error.

Among other causes, when gcc throws a warning before parsing any tokens,
the cur_token pointer is at the beginning of malloc'd memory.
Dereferencing cur_token[-1] can cause a segfault.

Code taken from OpenBSD
http://www.openbsd.org/cgi-bin/cvsweb/src/gnu/gcc/libcpp/errors.c
which was a more complete fix than the one I originally coded.

MFC after:	1 week