bignum: shift setint64 in unsigned space to avoid signed overflow UB

setint64() extracted big-endian bytes with a signed left shift (n <<= 8).
When the top byte is set that overflows int64 -- undefined behaviour, which
the script fuzzer flagged under UBSan (bignum.h:158). The result was correct
on two's-complement hardware, so this is a latent UB inherited from the
original 0.1 code, not a behaviour change. Do the shift on a uint64.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/bignum.h b/src/bignum.h
index 9da104b..6a295b2 100644
--- a/src/bignum.h
+++ b/src/bignum.h
@@ -155,7 +155,9 @@ public:
         for (int i = 0; i < 8; i++)
         {
             unsigned char c = (n >> 56) & 0xff;
-            n <<= 8;
+            n = (int64)((uint64)n << 8);  // shift in unsigned space; <<= on a
+                                          // signed int64 with the top byte set
+                                          // is overflow UB (fuzzer/UBSan finding)
             if (fLeadingZeroes)
             {
                 if (c == 0)