darwin-user: Remove unneeded null pointer check

cppcheck reports this error:

commpage.c:223: error: Possible null pointer dereference:
value - otherwise it is redundant to check if value is null at line 214

The null pointer check in line 214 is indeed not needed.
If value were null, the code would crash in line 223.
See do_compare_and_swap64 were for a reference.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
