Merge #9804: Fixes subscript 0 (&var[0]) where should use (var.data()) instead.

Summary:
30ac7688e Fix subscript[0] potential bugs in key.cpp (Jeremy Rubin)
4b1c0f2e2 Remove unnecessary branches in utilstrencodings string constructors. (Jeremy Rubin)
e19db7b5a Fix subscript[0] in utilstrencodings.cpp (Jeremy Rubin)
bc2e7fd98 Fix subscript[0] in streams.h (Jeremy Rubin)
4cac0d1e0 Fix subscript[0] in validation.cpp (Jeremy Rubin)
ac658e55f Fix subscript[0] in torcontrol (Jeremy Rubin)
b6856ebed Fix subscript[0] in netaddress.cpp (Jeremy Rubin)
361d95265 Fix subscript[0] in base58.cpp (Jeremy Rubin)
6896dbf16 Cleanup (safe, it was checked) subscript[0] in MurmurHash3 (and cleanup MurmurHash3 to be more clear). (Jeremy Rubin)
96f2119e6 Fix subscript[0] in compressor.cpp (Jeremy Rubin)
500710bd2 Fix 2 subscript[0] bugs in pubkey.cpp, and eliminate one extra size check (Jeremy Rubin)
e0451e3e2 Fix subscript[0] bug in net.cpp if GetGroup returns a 0-sized vector (Jeremy Rubin)

Tree-SHA512: 5b9103652cf8c615bd8f4f32b3573d291d6b67c39e0308ce00100bc6625f346e8e016b4c999f4f34f5c37ae059490a83c3b513deb21f838af785227d06e02362

Backport of Core PR 9693
https://github.com/bitcoin/bitcoin/pull/9804/

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D2964