libc: Remove NULL check for format argument in snprintf()
libc: Remove NULL check for format argument in snprintf()

libc printf style functions are annotated with __attribute__((format
(printf, x, y))) which causes GCC to perform compile time checks
against these arguments.

As of at least gcc 6.1.1 [(GCC) 6.1.1 20160602] this causes an error
running make check. GCC appears to be guaranteeing that the format
argument won't be NULL and complaining about explicit checks.

In file included from core/test/run-console-log-buf-overrun.c:48:0:
core/test/run-console-log-buf-overrun.c: In function ‘snprintf’:
core/test/../../libc/stdio/snprintf.c:21:19: error: nonnull argument
‘format’compared to NULL [-Werror=nonnull-compare]
if ((buff==NULL) || (format==NULL))
    ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors make:
*** [core/test/Makefile.check:59:core/test/run-console-log-buf-overrun] Error 1

Signed-off-by: Cyril Bur <6ce38dade8c4b52fd55c193fae70fd1d1275c3c3@au1.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

shenki/skiboot,mikey/skiboot,shenki/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,mikey/skiboot,open-power/skiboot,mikey/skiboot,qemu/skiboot,qemu/skiboot,legoater/skiboot,qemu/skiboot,stewart-ibm/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,legoater/skiboot,open-power/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot