x86: fix integer as NULL pointer warning

arch/x86/boot/printf.c:59:10: warning: Using plain integer as NULL pointer
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/boot/printf.c

@@ -56,7 +56,7 @@ static char *number(char *str, long num, int base, int size, int precision,
 	if (type & LEFT)
 		type &= ~ZEROPAD;
 	if (base < 2 || base > 36)
-		return 0;
+		return NULL;
 	c = (type & ZEROPAD) ? '0' : ' ';
 	sign = 0;
 	if (type & SIGN) {