Merge pull request #7 from gharris1727/fix-npe

Fix NPE when natives object is incomplete.

src/main/java/link/infra/jumploader/resolution/sources/MinecraftJarSource.java

@@ -157,7 +157,7 @@ public MetadataResolutionResult resolve(MetadataCacheHelper.MetadataCacheView ca
 
 				JsonObject downloadsObj = libObj.getAsJsonObject("downloads");
 				JsonObject nativesObj = libObj.getAsJsonObject("natives");
-				if (nativesObj != null) {
+				if (nativesObj != null && nativesObj.has(currentOS)) {
 					// TODO: should natives be ignored anyway? - we don't/can't handle them properly (see wiki)
 					String nativesClassifier = nativesObj.get(currentOS).getAsString();
 					JsonObject nativesDownloadObj = downloadsObj.getAsJsonObject("classifiers").getAsJsonObject(nativesClassifier);