strings.c: Fix __ast_str_helper() to always return a terminated string.

Users of functions which call __ast_str_helper() such as the ones listed
below are likely to not check the return value for failure so ensuring
that the string is always nil terminated is a good safety measure.

ast_str_set_va()
ast_str_append_va()
ast_str_set()
ast_str_append()

Change-Id: I36ab2d14bb6015868b49329dda8639d70fbcae07