Fixed crashes of 'ostream& operator<< (ostream& os, const char* s)' on NULL.

Fixed crashes of 'ostream& operator<< (ostream& os, const char* s)' on NULL.

Crashes observed on platforms OS X (clang-7.0.0) and FreeBSD-10.2 (clang-3.4).
First reported in bug #122 "clang does not compile re2c 0.15.x".

What caused NULL passed to 'operator <<': re2c always generates content of
header file (regardless of '-t --type-header' option), but the content is
dumped to file (and header filename initialized to non-NULL) only if the
option was enabled.

Fix: always initialize header filename to non-NULL string.