Check for array bounds limit while parsing pid length string

Ensure we never get an array overflow, fixes a potential issue
found by static analysis.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>

Author: ColinIanKing

faultstat.c

@@ -244,7 +244,7 @@ static int pid_max_digits(void)
 
 	buf[n] = '\0';
 	max_digits = 0;
-	while (buf[max_digits] >= '0' && buf[max_digits] <= '9')
+	while ((max_digits < n) && (buf[max_digits] >= '0') && (buf[max_digits] <= '9'))
 		max_digits++;
 	if (max_digits < min_digits)
 		max_digits = min_digits;