Avoid implicit-integer-sign-change in bech32.cpp

test/sanitizer_suppressions/ubsan

@@ -53,7 +53,6 @@ unsigned-integer-overflow:policy/fees.cpp
 unsigned-integer-overflow:prevector.h
 unsigned-integer-overflow:script/interpreter.cpp
 unsigned-integer-overflow:txmempool.cpp
-implicit-integer-sign-change:bech32.cpp
 implicit-integer-sign-change:compat/stdin.cpp
 implicit-integer-sign-change:compressor.h
 implicit-integer-sign-change:crypto/