pkproto.c: fix use-after-free that did cause pagekite to SIGSEGV

[plan44]

• occasionally, with higher probability on faster/multicore systems
• like: never crashed in years on RPi B+, crashed 95% of all startup attempts on Rpi3 or Rpi4

[plan44]

Some more context:

Observation:

• programs using libpagekitec did crash with SIGSEGV on startup on RPi3,4 while having worked fine for years on RPi1+2

Explantation:

• the final "judgement" test were done on pointers into the copy buffer freed on line 766 instead of on the safe copies of those strings in kite and kite_r.
• this opened a very short race condition window, however the crash was caught happening while a tight loop (pkb_start_blockers) fired up 16 of those threads in rapid succession. So probably if the next thread got to allocate memory before the "judgements" tests, accessing the just freed copy would cause a segfault.

Fix:

• check the safe copies of the strings instead of pointers into copy.