bigint.c (mpz_get_int): avoid buffer overflow.

mrbgems/mruby-bigint/core/bigint.c

@@ -742,17 +742,21 @@ static int
 mpz_get_int(mpz_t *y, mrb_int *v)
 {
   mp_limb i;
+  size_t d;
 
   if (y->sn == 0) {
     i = 0;
   }
-  else if (digits(y) > 2 || y->p[1] > 1) {
+  else if ((d = digits(y)) > 2 || (d == 2 && y->p[1] > 1)) {
     return FALSE;
   }
-  else {
+  else if (d == 2) {
     i = (y->sn * (y->p[0] | (y->p[1] & 1) << DIGITBITS));
-    if (MRB_INT_MAX < i || i < MRB_INT_MIN) return FALSE;
   }
+  else {/* d == 1 */
+    i = y->sn * y->p[0];
+  }
+  if (MRB_INT_MAX < i || i < MRB_INT_MIN) return FALSE;
   *v = i;
   return TRUE;
 }