Avoid signed integer overflow when loading a mempool.dat file with a malformed time field
Summary: This is a backport of core#20372
Test Plan:
This causes an error before applying the commit, and it works after the change:
$ cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DENABLE_SANITIZERS=undefined $ xxd -p -r > mempool.dat-crash-1 <<EOF 0100000000000000000000000004000000000000000000000000ffffffff ffffff7f00000000000000000000000000 EOF $ cp mempool.dat-crash-1 /bitcoinddata/regtest/mempool.dat $ ninja $ UBSAN_OPTIONS="print_stacktrace=1:halt_on_error=1:report_error_type=1" src/bitcoind -regtest
../src/validation.cpp:5853:23: runtime error: signed integer overflow: 9223372036854775807 + 1209600 cannot be represented in type 'long'
#0 0x55f14103ffcd in LoadMempool(Config const&, CTxMemPool&) /home/pierre/dev/bitcoin-abc/build_ubsan/../src/validation.cpp:5853:23
#1 0x55f14103fb65 in CChainState::LoadMempool(Config const&, ArgsManager const&) /home/pierre/dev/bitcoin-abc/build_ubsan/../src/validation.cpp:4821:9
...Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10725