Minor cleanup: make brk_string to return argc only if the pointer to it

Minor cleanup: make brk_string to return argc only if the pointer to it
was non-NULL. This let's us eliminated an otherwise unused variable.

shellneeded can never return -1 so there is no need to check for it and
hence no need for a variable to hold the returned value.

Submitted by:	Max Okumoto <okumoto@ucsd.edu> (partly)