If Agent_OnAttach() in JVMTI agent which is attempted to load via JVMTI.agent_load dcmd is failed, it would not be unloaded.

How to reproduce:

1. Build JVMTI agent for test
  $ git clone https://github.com/YaSuenag/jvmti-examples.git
  $ cd jvmti-examples/helloworld/out/build
  $ cmake ../..

2. Run JShell

3. Load JVMTI agent via `jcmd <PID> JVMTI.agent_load` with "error" ("error" means Agent_OnAttach() returns JNI_ERR)

  $ jcmd
  89456 jdk.jshell.execution.RemoteExecutionControl 45651
  89547 sun.tools.jcmd.JCmd
  89436 jdk.jshell/jdk.internal.jshell.tool.JShellToolProvider
  $ jcmd 89436 JVMTI.agent_load `pwd`/libhelloworld.so error
  89436:
  return code: -1

4. Check loaded libraries via `jcmd <PID> VM.dynlibs`

$ jcmd 89436 VM.dynlibs | grep libhelloworld
7f2f8b06b000-7f2f8b06c000 r--p 00000000 fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so
7f2f8b06c000-7f2f8b06d000 r-xp 00001000 fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so
7f2f8b06d000-7f2f8b06e000 r--p 00002000 fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so
7f2f8b06e000-7f2f8b06f000 r--p 00002000 fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so
7f2f8b06f000-7f2f8b070000 rw-p 00003000 fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so