Do not index past end of buffer when checking heuristic in error index syntax highlighter

[estebank]

When checking whether the current token is a function indentifier by inspecting the syntax, do not attempt to access past the end of the code buffer.

Fix #156326.

[rustbot]

r? @petrochenkov

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[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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fmt check
Diff in /checkout/tests/run-make/explain-output/rmake.rs:7:
 
 fn main() {
     let out = rustc().args(&["--explain=E0602", "--color=always"]).run().stdout_utf8();
-    diff()
-        .expected_file("explain-E0602.stderr")
-        .actual_text("(rustc)", &out)
-        .run();
+    diff().expected_file("explain-E0602.stderr").actual_text("(rustc)", &out).run();
 }
 
fmt: checked 6853 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:48

[petrochenkov]

r=me after fixing tidy.
@rustbot author

[rustbot]

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