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/gui/skin_engine/skin_tokens.c

@@ -263,7 +263,7 @@ const char *get_id3_token(struct wps_token *token, struct mp3entry *id3,
             case SKIN_TOKEN_METADATA_TRACK_NUMBER:
                 if (id3->track_string)
                     return id3->track_string;
-                if (id3->tracknum) {
+                if (id3->tracknum >= 0) {
                     itoa_buf(buf, buf_size, id3->tracknum);
                     return buf;
                 }

apps/plugins/lib/mul_id3.c

@@ -173,7 +173,7 @@ void finalize_id3(struct mp3entry *id3)
     id3->codectype = mul_id3.codectype;
     id3->vbr = mul_id3.vbr;
     id3->discnum = 0;
-    id3->tracknum = 0;
+    id3->tracknum = -1;
     id3->track_level = 0;
     id3->album_level = 0;
 }

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;

apps/tagcache.c

@@ -2322,7 +2322,7 @@ static void NO_INLINE add_tagcache(char *path, unsigned long mtime)
 
     logf("-> %s", path);
 
-    if (id3.tracknum <= 0)              /* Track number missing? */
+    if (id3.tracknum < 0)              /* Track number missing? */
     {
         id3.tracknum = -1;
     }