internal/lsp: add nil check for control flow highlighting
internal/lsp: add nil check for control flow highlighting

Added a check to make sure that highlighting the control flow of
a function only continues if the cursor is actually in a function.

Change-Id: Idac90d9e55c09c3dcb9ae938585157658acc95e9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/209581
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Run-TryBot: Rohan Challa <9671c73253cee9b18b8df367e3e627fab2863fe8@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools