Fix possible BannerHandler NPE in 1.13->1.12.2 (#814)

common/src/main/java/com/viaversion/viabackwards/protocol/v1_13to1_12_2/block_entity_handlers/BannerHandler.java

@@ -51,9 +51,11 @@ public class BannerHandler implements BackwardsBlockEntityHandler {
         if (patternsTag != null) {
             for (CompoundTag pattern : patternsTag) {
                 NumberTag colorTag = pattern.getNumberTag("Color");
+                if (colorTag != null) {
                     pattern.putInt("Color", 15 - colorTag.asInt()); // Invert color id
                 }
             }
+        }
 
         return tag;
     }