script: prevent UB when computing abs value for num opcode serialize

Author: pierrenn

src/script/script.h

@@ -329,7 +329,7 @@ class CScriptNum
 
         std::vector<unsigned char> result;
         const bool neg = value < 0;
-        uint64_t absvalue = neg ? -value : value;
+        uint64_t absvalue = neg ? ~static_cast<uint64_t>(value) + 1 : static_cast<uint64_t>(value);
 
         while(absvalue)
         {

src/test/fuzz/integer.cpp

@@ -135,11 +135,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
 
     const CScriptNum script_num{i64};
     (void)script_num.getint();
-    // Avoid negation failure:
-    // script/script.h:332:35: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself
-    if (script_num != CScriptNum{std::numeric_limits<int64_t>::min()}) {
-        (void)script_num.getvch();
-    }
+    (void)script_num.getvch();
 
     const arith_uint256 au256 = UintToArith256(u256);
     assert(ArithToUint256(au256) == u256);

src/test/fuzz/scriptnum_ops.cpp

@@ -129,10 +129,6 @@ void test_one_input(const std::vector<uint8_t>& buffer)
             break;
         }
         (void)script_num.getint();
-        // Avoid negation failure:
-        // script/script.h:332:35: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself
-        if (script_num != CScriptNum{std::numeric_limits<int64_t>::min()}) {
-            (void)script_num.getvch();
-        }
+        (void)script_num.getvch();
     }
 }