WGSL front end crashes on malformed input

[GabrielMajeri]

Minimal reproduction: feed the WGSL parser the following string:

"\"\u{2}ПЀ\u{0}\""

Leads to the following panic in wgsl::lex::consume_token:

thread '<unnamed>' panicked at 'byte index 4 is not a char boundary; it is inside 'Ѐ' (bytes 3..5) of `ПЀ"`', /home/gabriel/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/str/mod.rs:1987:47
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: libfuzzer_sys::initialize::{{closure}}
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:524
  12: rust_begin_unwind
             at src/libstd/panicking.rs:431
  13: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  14: core::str::slice_error_fail
             at src/libcore/str/mod.rs:0
  15: core::str::traits::<impl core::slice::SliceIndex<str> for core::ops::range::RangeTo<usize>>::index::{{closure}}
  16: naga::front::wgsl::lex::consume_token
  17: naga::front::wgsl::Parser::parse_global_decl
  18: naga::front::wgsl::Parser::parse
  19: rust_fuzzer_test_input
  20: __rust_try
  21: LLVMFuzzerTestOneInput
  22: _ZN6fuzzer6Fuzzer15ExecuteCallbackEPKhm
  23: _ZN6fuzzer10RunOneTestEPNS_6FuzzerEPKcm
  24: _ZN6fuzzer12FuzzerDriverEPiPPPcPFiPKhmE
  25: main
  26: __libc_start_main
  27: _start

The panic is likely from this indexing here:

naga/src/front/wgsl.rs

Line 101 in 1f5008f

(Token::String(&base[..len]), chars.as_str())