TagFile: Fix off-by-one errors in comment stripping

Adding 1 to the value of d->End - current makes restLength one byte
too long: If we pass memchr(current, ..., restLength) has thus
undefined behavior.

Also, reading the value of current has undefined behavior if
current >= d->End, not only for current > d->End:

Consider a string of length 1, that is d->End = d->Current + 1.
We can only read at d->Current + 0, but d->Current + 1 is beyond
the end of the string.

This probably caused several inexplicable build failures on hurd-i386
in the past, and just now caused a build failure on Ubuntu's amd64
builder.

Reported-By: valgrind