Fix NULL dereference in "-S name" when syscall table has holes

* count.c (syscall_cmp): Do not pass NULL to strcmp.
diff --git a/count.c b/count.c
index 4b16ee6..1393b6e 100644
--- a/count.c
+++ b/count.c
@@ -112,8 +112,9 @@
 static int
 syscall_cmp(void *a, void *b)
 {
-	return strcmp(sysent[*((int *) a)].sys_name,
-		      sysent[*((int *) b)].sys_name);
+	const char *a_name = sysent[*((int *) a)].sys_name;
+	const char *b_name = sysent[*((int *) b)].sys_name;
+	return strcmp(a_name ? a_name : "", b_name ? b_name : "");
 }
 
 static int