skin_engine: Fix %if() when comparing against a negative number.

If the tag being checked returns a negative number (e.g. %LI does that
when the current list item does not have an icon) the tag's value would
always end up being 1.

Change-Id: I69ab175b6c46dd7567386e649732dbb8327972fc

apps/gui/skin_engine/skin_tokens.c

@@ -598,7 +598,7 @@ static const char* NOINLINE get_lif_token_value(struct gui_wps *gwps,
             break;
         }
         case INTEGER:
-            if (!number_set && out_text && *out_text >= '0' && *out_text <= '9')
+            if (!number_set && out_text && ((*out_text >= '0' && *out_text <= '9') || (*out_text == '-' && out_text[1] >= '0' && out_text[1] <= '9')))
                 a = atoi(out_text);
             /* fall through */
         case PERCENT:

docs/CREDITS

@@ -699,6 +699,7 @@ Michael Rey
 Yuxuan Shui
 James Buren
 Issa Beganović
+Georg Gadinger
 
 The libmad team
 The wavpack team