FS#13497: '0' is sometimes a valid track number

Most notably for CD rips that use a track number of 0 for
the leadin.

Therefore change our "invalid track number" canary to -1 instead
of 0.  Additionally don't try to parse an empty string.

In the process, get rid of redudant 'discnum = 0' as well.

NOTE: While not strictly necessary, we recommend rebuilding the
      database to ensure files without track numbers are
      updated with the new canary.

Change-Id: I543f98ca49cec7b5eeffa7c14c1eca57171f345a

apps/screens.c

@@ -571,7 +571,7 @@ static const char * id3_get_or_speak_info(int selected_item, void* data,
                     if(say_it)
                         say_number_and_spell(val, true);
                 }
-                else if (id3->tracknum)
+                else if (id3->tracknum >= 0)
                 {
                     itoa_buf(buffer, buffer_len, id3->tracknum);
                     val = buffer;