Fix NPE in ContextSelector.getContext() (apache#1538 LOG4J2-3217)

Author: pbodnar

log4j-core/src/main/java/org/apache/logging/log4j/core/selector/ContextSelector.java

@@ -108,7 +108,7 @@ default LoggerContext getContext(String fqcn, ClassLoader loader, Map.Entry<Stri
     default LoggerContext getContext(String fqcn, ClassLoader loader, Map.Entry<String, Object> entry,
             boolean currentContext, URI configLocation) {
         final LoggerContext lc = getContext(fqcn, loader, currentContext, configLocation);
-        if (lc != null) {
+        if (lc != null && entry != null) {
             lc.putObject(entry.getKey(), entry.getValue());
         }
         return lc;