Merge #10341: rpc/wallet: Workaround older UniValue which returns a std::string temporary for get_str

Summary:
a637734 rpc/wallet: Workaround older UniValue which returns a std::string temporary for get_str (Luke Dashjr)

Tree-SHA512: 8f03f1d301f714f700bf64e259a1c986136fd1ac532f091aa97198a39c131290c320094f4fc38c774c8db4469a75437596b1d933300e4f04037abc158a252bad

Backport of Core PR10341
https://github.com/bitcoin/bitcoin/pull/10341/files

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, markblundeberg, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3106