The counter will only ever be a valid u32 value in normal operation, so document that without changing any behavior.
In theory, the behavior could be changed to use LocaleIndependentAtoi<uint32_t>(), but this is left for the future.
The change is needed to be able to fuzz the rpc target.
To test after compiling with -DSANITIZERS=undefined,integer,float-divide-by-zero:
$ base64 /tmp/a.crash
Z2V0YmxvY2t0ZW1wbGF0ZVyzXC1sb25ncG9sbGlkXNHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR
AAAAAAAAAAAAAAAgAAAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAABCQkJCQkJCQkJCQkJCQkJCQkJC
QkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJC
QkJCQkJCQkJCQkJCQkLR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHR
0dHR0dHR0dHR0dHR0dHR0dHR0dHR0dHRZ2VsA6NjA2+pYysAmfcAuMXhQwjp7WJsXA==
$ FUZZ=rpc ./bld-cmake/test/with_sanitizer_env.py ./bld-cmake/bin/fuzz /tmp/a.crash
src/rpc/mining.cpp:842:42: runtime error: implicit conversion from type 'long' of value 9223372036854775807 (64-bit, signed) to type 'unsigned int' changed the value to 4294967295 (32-bit, unsigned)
[#0](/bitcoin-bitcoin/0/) 0x5603ba69a874 in getblocktemplate()::$_0::operator()(RPCMethod const&, JSONRPCRequest const&) const
...
Or:
$ UBSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=0:report_error_type=1" ./bld-cmake/bin/bitcoind -datadir=/tmp -regtest &
$ ./bld-cmake/bin/bitcoin-cli -datadir=/tmp -regtest getblocktemplate '{"rules":["segwit"],"longpollid":"00000000000000000000000000000000000000000000000000000000000000009223372036854775807"}'
