parseopt: remove unreachable code

parseopt: remove unreachable code

At this point in the code after running skip_prefix() on the
variable and receiving the result in the same variable, the contents
of the variable can never be NULL.  The function either (1) updates
the variable to point at a later part of the string it originally
pointed at, or (2) leaves it intact if the string does not have the
prefix.  (1) will never make the variable NULL, and (2) cannot be
the source of NULL, because the variable cannot be NULL before
calling skip_prefix(), which would die immediately by dereferencing
the NULL pointer in that case.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>