fix compiler warning

bn_mp_set_long.c

@@ -18,10 +18,11 @@
 /* set a platform dependent unsigned long int */
 int mp_set_long (mp_int * a, unsigned long b)
 {
-  int     x, res;
+  unsigned int  x;
+  int           res;
 
   mp_zero (a);
-  
+
   /* set four bits at a time */
   for (x = 0; x < sizeof(unsigned long) * 2; x++) {
     /* shift the number up four bits */