color_string_internal to use a sentinel value that's definitely invalid

I think -1 is slightly more elegant than 0 because 0 could be a valid offset.

No functional change.

src/highlight.cpp

@@ -552,7 +552,7 @@ static void color_string_internal(const wcstring &buffstr, highlight_spec_t base
 
     enum { e_unquoted, e_single_quoted, e_double_quoted } mode = e_unquoted;
     // For some ungodly reason making this a maybe<size_t> makes gcc grumpy
-    size_t unclosed_quote_offset = 0;
+    auto unclosed_quote_offset = std::numeric_limits<size_t>::max();
     int bracket_count = 0;
     for (size_t in_pos = 0; in_pos < buff_len; in_pos++) {
         const wchar_t c = buffstr.at(in_pos);