lib/gshadow.c: build_list(): Allocate at once

lib/gshadow.c: build_list(): Allocate at once

Instead of reallocating 1 more meber per iteration, calculate the total
amount that we want by counting the number of commas (delimiters) in the
string, plus one for the last element, plus one for the terminating
NULL.

This might result in overallocation of one element if the string is an
empty string, or if there's a trailing comma; however, that's not an
issue.  We can afford overallocating one element in certain cases, and
we get in exchange a much simpler function.

Signed-off-by: Alejandro Colomar <alx@kernel.org>