AbstractBootstrap can crash instead of failing promise, close #5387

Motivation:
When `ChannelFactory#newChannel` crashed, `AbstractBootstrap#initAndRegister` propagates the exception to the caller instead of failing the promise.

Modifications:
- Catch exceptions from `ChannelFactory#newChannel`.
- Notify promise of such failure.

Result:
`AbstractBootstrap` gracefully handles connect failures.

transport/src/main/java/io/netty/bootstrap/AbstractBootstrap.java

@@ -304,11 +304,15 @@ public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C ext
     }
 
     final ChannelFuture initAndRegister() {
-        final Channel channel = channelFactory().newChannel();
+        Channel channel = null;
         try {
+            channel = channelFactory().newChannel();
             init(channel);
         } catch (Throwable t) {
-            channel.unsafe().closeForcibly();
+            if (channel != null) {
+                // channel can be null if newChannel crashed (eg SocketException("too many open files"))
+                channel.unsafe().closeForcibly();
+            }
             // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
             return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
         }

transport/src/test/java/io/netty/bootstrap/BootstrapTest.java

@@ -216,6 +216,7 @@ public class BootstrapTest {
     }
 
     private static final class TestLocalEventLoopGroup extends LocalEventLoopGroup {
+
         ChannelPromise promise;
         TestLocalEventLoopGroup() {
             super(1);