civetweb.c: use NULL for pointers
civetweb.c: use NULL for pointers

fix for cgcc warnings:

src/civetweb.c:5455:56: warning: Using plain integer as NULL pointer
src/civetweb.c:5696:28: warning: Using plain integer as NULL pointer
src/civetweb.c:5698:26: warning: Using plain integer as NULL pointer
src/civetweb.c:6456:34: warning: Using plain integer as NULL pointer
src/civetweb.c:6643:34: warning: Using plain integer as NULL pointer
src/civetweb.c:6811:29: warning: Using plain integer as NULL pointer

Signed-off-by: Danny Al-Gaaf <473e39ac77cf392f0dd9bacde8409de76c14d006@bisect.de>

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver