doc: hpfall.c: fix missing null-terminate after strncpy call

Added a guaranteed null-terminate after call to strncpy.

This was partly found using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>