internal/lsp: add an additional check to ToUTF16Column

This change will at least stop the panic from occurring.

Updates golang/go#31341

Change-Id: I6b06941cfb6c020d2b37813573cb0dc068d54e65
Reviewed-on: https://go-review.googlesource.com/c/tools/+/172667
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/span/utf16.go

@@ -35,9 +35,14 @@ func ToUTF16Column(p Point, content []byte) (int, error) {
 	if lineOffset < 0 || offset > len(content) {
 		return -1, fmt.Errorf("ToUTF16Column: offsets %v-%v outside file contents (%v)", lineOffset, offset, len(content))
 	}
-	// use the offset to pick out the line start
+	// Use the offset to pick out the line start.
+	// This cannot panic:  offset > len(content) and lineOffset < offset.
 	start := content[lineOffset:]
-	// now truncate down to the supplied column
+
+	// Now, truncate down to the supplied column.
+	if col >= len(start) {
+		return -1, fmt.Errorf("ToUTF16Column: line (%v) is shorter than column (%v)", len(start), col)
+	}
 	start = start[:col]
 	// and count the number of utf16 characters
 	// in theory we could do this by hand more efficiently...