Fix tput when compiled with clang-15 -O2

Fix tput when compiled with clang-15 -O2

For some reason clang-15 doesn't like passing the uninitialized array of
pointers nargv[] to the vararg function tparm(). With -O2 it optimizes the
for loop preceding the tparm() call strangely, with the result that the
argv[i] == NULL error is hit in most real-world usage. This broke naddy's
fancy shell prompt among other things. Initialize nargv[] to appease the
insatiable undefined behavior exploiter.

ok jca millert