The sequence
#define m()
int m"
creates a token of no length (a string of 0 size). Protect
against a string of 0 size as well as the existing protect
against a null string.
This would look ahead for a second #, for token pasting, and if not
found, backup one token. This is fine, unless at the end of line,
which would backup the #, rather than the look ahead.