argv_split: allow argv_split to handle NULL pointer in argcp parameter gracefully

It would be nice if the argv_split library function could gracefully handle
a NULL pointer in the argcp parameter, so as to allow functions using it
that did not care about the value of argc to not have to declare a useless
variable.  This patch accomplishes that.  Tested by me, with successful
results.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>