Merge #18413: script: prevent UB when computing abs value for num opcode serialize

2748e8793267126c5b40621d75d1930e358f057e script: prevent UB when computing abs value for num opcode serialize (pierrenn)

Pull request description:

  This was reported by practicalswift here #18046

  It seems that the original author of the line used a reference to glibc `abs`: https://github.com/lattera/glibc/blob/master/stdlib/abs.c

  However depending on some implementation details this can be undefined behavior for unusual values.

  A detailed explanation of the UB is provided here : https://stackoverflow.com/questions/17313579/is-there-a-safe-way-to-get-the-unsigned-absolute-value-of-a-signed-integer-with (by [Billy O'Neal](https://twitter.com/malwareminigun))

  Simple relevant godbolt example :  https://godbolt.org/z/yRwtCG

  Thanks!

ACKs for top commit:
  sipa:
    ACK 2748e8793267126c5b40621d75d1930e358f057e
  MarcoFalke:
    ACK 2748e8793267126c5b40621d75d1930e358f057e, only checked that the bitcoind binary does not change with clang -O2 🎓
  practicalswift:
    ACK 2748e8793267126c5b40621d75d1930e358f057e

Tree-SHA512: 539a34c636c2674c66cb6e707d9d0dfdce63f59b5525610ed88da10c9a8d59d81466b111ad63b850660cef3750d732fc7755530c81a2d61f396be0707cd86dec

1 files changed, 1 insertions, 1 deletions

diff --git a/src/script/script.h b/src/script/script.h
index daf4224530..773ffbb985 100644
--- a/src/script/script.h
+++ b/src/script/script.h
@@ -329,7 +329,7 @@ public:
 
         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)
         {