FuzzCSSParser currently crashes on any input it is given.

Can be reproduced with:

> ./Meta/Lagom/BuildFuzzers.sh
> ./Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser

which results in the following stack trace:

    0x7fc71a7d55d8 in ak_verification_failed /home/tim/repos/serenity/AK/Assertions.cpp:106:5
    0x559a98ffb120 in AK::Vector<JS::ExecutionContext*, 0ul>::at(unsigned long) /home/tim/repos/serenity/Meta/Lagom/../../AK/Vector.h:148:9
    0x559a98ffb120 in AK::Vector<JS::ExecutionContext*, 0ul>::last() /home/tim/repos/serenity/Meta/Lagom/../../AK/Vector.h:162:34
    0x559a98ffac9d in JS::VM::running_execution_context() /home/tim/repos/serenity/Meta/Lagom/../../Userland/Libraries/LibJS/Runtime/VM.h:114:87
    0x559a98ffac9d in JS::VM::current_realm() /home/tim/repos/serenity/Meta/Lagom/../../Userland/Libraries/LibJS/Runtime/VM.h:131:37
    0x559a98ffac9d in LLVMFuzzerTestOneInput /home/tim/repos/serenity/Meta/Lagom/Fuzzers/FuzzCSSParser.cpp:29:74
    0x559a98e9ea78 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/tim/repos/serenity/Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser+0x57a78) (BuildId: 739f6ddedcfe7f3e13ab8f19f820a4401597fe19)
    0x559a98ea0d25 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) (/home/tim/repos/serenity/Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser+0x59d25) (BuildId: 739f6ddedcfe7f3e13ab8f19f820a4401597fe19)
    0x559a98ea13fe in fuzzer::Fuzzer::Loop(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) (/home/tim/repos/serenity/Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser+0x5a3fe) (BuildId: 739f6ddedcfe7f3e13ab8f19f820a4401597fe19)
    0x559a98e81af2 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/tim/repos/serenity/Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser+0x3aaf2) (BuildId: 739f6ddedcfe7f3e13ab8f19f820a4401597fe19)
    0x559a98e6b557 in main (/home/tim/repos/serenity/Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser+0x24557) (BuildId: 739f6ddedcfe7f3e13ab8f19f820a4401597fe19)
    0x7fc71a245ccf  (/usr/lib/libc.so.6+0x27ccf) (BuildId: 8bfe03f6bf9b6a6e2591babd0bbc266837d8f658)
    0x7fc71a245d89 in __libc_start_main (/usr/lib/libc.so.6+0x27d89) (BuildId: 8bfe03f6bf9b6a6e2591babd0bbc266837d8f658)
    0x559a98e6b894 in _start (/home/tim/repos/serenity/Meta/Lagom/Build/lagom-fuzzers/bin/FuzzCSSParser+0x24894) (BuildId: 739f6ddedcfe7f3e13ab8f19f820a4401597fe19)