Fix FuzzRPCClientPartialLength test (MSSQL Server)

[gabrielcorado]

The test failed with:

$ go test -run=^$ ./lib/srv/db/sqlserver/protocol -fuzz=FuzzRPCClientPartialLength
fuzz: elapsed: 0s, gathering baseline coverage: 0/59 completed
failure while testing seed corpus entry: FuzzRPCClientPartialLength/b5768bce0ade2442
fuzz: elapsed: 2s, gathering baseline coverage: 0/59 completed
--- FAIL: FuzzRPCClientPartialLength (2.12s)
    --- FAIL: FuzzRPCClientPartialLength (0.00s)
        fuzz_test.go:62:
                Error Trace:    teleport/teleport/lib/srv/db/sqlserver/protocol/fuzz_test.go:62
                                                        /opt/homebrew/Cellar/go/1.20.2/libexec/src/reflect/value.go:586
                                                        /opt/homebrew/Cellar/go/1.20.2/libexec/src/reflect/value.go:370
                                                        /opt/homebrew/Cellar/go/1.20.2/libexec/src/testing/fuzz.go:335
                Error:          Received unexpected error:
                                failed to convert packet to SQL packet: runtime error: invalid memory address or nil pointer dereference
                Test:           FuzzRPCClientPartialLength

FAIL
exit status 1
FAIL    github.com/gravitational/teleport/lib/srv/db/sqlserver/protocol 2.761s

Thanks, @Tener, for pointing it out!

The problem was related to the packet fixture used on this test, it had two errors:

• Packet length was not properly encoded and limited the packet size.
• The packet did not contain all the data if the length was not divisible by the chunks.

Both were addressed in this PR.

Note: No production code changes.

[Tener]

Good stuff 👍

We may want to add FuzzRPCClientPartialLength to fuzz/oss-fuzz-build.sh so that it gets covered by OSS-Fuzz project.

The changes are rather straightforward, but if you want to test them you should do:

> git clone --depth=1 https://github.com/google/oss-fuzz.git
> cd oss-fuzz
> python3 infra/helper.py build_fuzzers teleport ~/path/to/repo/teleport

Note:

• the build process leaves some changes to the repo under ~/path/to/repo/teleport.
• on M1 you should make Docker use Rosetta emulation or the build will take hours.