civetweb.c: check for array boundry before read

Added assert to while loop to make sure not to read over the array boundry.

Fix for Coverity issue from Ceph project:

CID 1160832 (#1 of 1): Out-of-bounds read (OVERRUN_STATIC)
 37. overrun-local: Overrunning static array "buf", with 8192
 elements, at position 8192 with index variable "len".

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
main.c: move 'guard' variable to _WIN32

Only needed in _WIN32 case. Otherwise unused.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>