Merge #14510: Avoid triggering undefined behaviour in base_uint<BITS>…

…::bits()

Summary:
PR14510 backport https://github.com/bitcoin/bitcoin/pull/14510/files
96f6dc9fc50b1cc59e26d50940ebf46e1bdcc0ba Avoid triggering undefined behaviour in base_uint<BITS>::bits() (practicalswift)

Pull request description:

  Avoid triggering undefined behaviour in `base_uint<BITS>::bits()`.

  `1 << 31` is undefined behaviour in C++11.

  Given the reasonable assumption of `sizeof(int) * CHAR_BIT == 32`.

Test Plan: `make check`

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3126

src/arith_uint256.cpp

@@ -152,7 +152,7 @@ template <unsigned int BITS> unsigned int base_uint<BITS>::bits() const {
     for (int pos = WIDTH - 1; pos >= 0; pos--) {
         if (pn[pos]) {
             for (int nbits = 31; nbits > 0; nbits--) {
-                if (pn[pos] & 1 << nbits) {
+                if (pn[pos] & 1U << nbits) {
                     return 32 * pos + nbits + 1;
                 }
             }