updated undefined behavior in accordance with zcash remedy rex4539 #4056

src/arith_uint256.cpp

@@ -193,9 +193,10 @@ unsigned int base_uint<BITS>::bits() const
 {
     for (int pos = WIDTH - 1; pos >= 0; pos--) {
         if (pn[pos]) {
-            for (int bits = 31; bits > 0; bits--) {
-                if (pn[pos] & 1 << bits)
+            for (size_t bits = 31; bits > 0; bits--) {
+                if (pn[pos] & (1U << bits)) {
                     return 32 * pos + bits + 1;
+				}
             }
             return 32 * pos + 1;
         }