Fix segmentation fault when showing help text.
Found when running dvbv5-scan (part of coming v4l-utils upgrade).
Backtrace:
#0 hol_entry_help (entry=0x8007e0000, state=0x0, stream=0x8007d3000,
hhstate=0x7fffffffe5d8) at argp-help.c:1164
#1 hol_help (hol=0x8007df000, state=0x0, stream=0x8007d3000) at argp-help.c:1230
#2 _help (argp=0x7fffffffe750, state=0x0, stream=0x8004fca20, flags=634,
0x2018cedvbv5-scan) at argp-help.c:1675
#3 argp_help (argp=0x7fffffffe750, stream=0x8004fca20, flags=634,
0x2018cedvbv5-scan) at argp-help.c:1707
Upstream maintainer has been notified.
Approved by: pi (implicit)