fix: gcc-11 workaround for memcmp in uint256 class

This workaround let enable -Werror -Wall on our CI

    In member function ‘constexpr int base_blob<BITS>::Compare(const base_blob<BITS>&) const [with unsigned int BITS = 256]’,
        inlined from ‘constexpr bool operator!=(const base_blob<256>&, const base_blob<256>&)’ at ./uint256.h:58:96,
        inlined from ‘bool CDeterministicMNManager::BuildNewListFromBlock(const CBlock&, gsl::not_null<const CBlockIndex*>, BlockValidationState&, const CCoinsViewCache&, CDeterministicMNList&, llmq::CQuorumSnapshotManager&, bool)’ at evo/deterministicmns.cpp:818:119:
    ./uint256.h:55:77: error: ‘int __builtin_memcmp_eq(const void*, const void*, long unsigned int)’ specified bound 32 exceeds source size 0 [-Werror=stringop-overread]
       55 |     constexpr int Compare(const base_blob& other) const { return std::memcmp(m_data.data(), other.m_data.data(), WIDTH); }
          |                                                                  ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cc1plus: all warnings being treated as errors

Author: knst

src/uint256.h

@@ -52,7 +52,19 @@ class base_blob
         std::fill(m_data.begin(), m_data.end(), 0);
     }
 
-    constexpr int Compare(const base_blob& other) const { return std::memcmp(m_data.data(), other.m_data.data(), WIDTH); }
+    // TODO: remove this gcc-11 workaround for memcmp
+    // gcc-11 can not recognise size of internal data in std::array even if width is not zero and show this warning:
+    // error: ‘int __builtin_memcmp_eq(const void*, const void*, long unsigned int)’ specified bound 32 exceeds source size 0 [-Werror=stringop-overread]
+#if (defined __GNUC__) && (__GNUC__ < 12)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-overread"
+#endif
+    constexpr int Compare(const base_blob& other) const {
+        return std::memcmp(m_data.data(), other.m_data.data(), WIDTH);
+    }
+#if (defined __GNUC__) && (__GNUC__ < 12)
+#pragma GCC diagnostic pop
+#endif
 
     friend constexpr bool operator==(const base_blob& a, const base_blob& b) { return a.Compare(b) == 0; }
     friend constexpr bool operator!=(const base_blob& a, const base_blob& b) { return a.Compare(b) != 0; }