BeamHash III: don't read past the end of the index vector (#2151)

[profinch]

Fixes #2151.

GetMinimalFromIndices started its loop at i = sol.size() and so read sol[32], one element past the end. The loop now starts at sol.size() - 1.

The output is unchanged; the stray element was shifted out of the 800-bit stream anyway.

Tested

• Old vs fixed function on 100,000 random 32-index sets: byte-identical output, and GetIndicesFromMinimal round-trips every fixed result. Under _LIBCPP_HARDENING_MODE_DEBUG the old version aborts at once (vector[] index out of bounds); the fixed one runs clean under ASan/UBSan too.
• The reference solver (OptimisedSolve) built with this change, input = 32 zero bytes, nonce = 0: the same solution as before (one solution, 5 min on an M5 Pro).
• equihash_test, stratum_test pass (macOS arm64, RelWithDebInfo).