Mended Color Mixer random color bug

If you set a color, then come back to Color Mixer and immediately
dismiss the dialog (without setting a color), an indeterminate
color could appear due to uninitialized variables (new_r, _g, _b).

Closes https://sourceforge.net/p/tuxpaint/bugs/251/
h/t @kentonyanamin on Twitter

src/tuxpaint.c

@@ -23209,6 +23209,14 @@ static int do_color_mix(void)
       for (i = 0; i < NUM_MIXER_COLORS; i++)
         color_mixer_color_counts[i] = 0;
     }
+  else
+    {
+      new_r = color_hexes[COLOR_MIXER][0];
+      new_g = color_hexes[COLOR_MIXER][1];
+      new_b = color_hexes[COLOR_MIXER][2];
+    }
+
+
 
   done = 0;
   chose = 0;