plugin: call dlclose if loading a plugin fails

[wangzz719]

What version of Go are you using (go version)?

$ go version
go version go1.12.5 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/wangzhizhao/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/wangzhizhao/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build948496286=/tmp/go-build -gno-record-gcc-switches"

What did you do?

First I load an .so built with go1.12，while error occur，I load another .so built with go1.12.5，still got error：plugin was built with a different version of package errors。

If I only load the .so which built with go1.12.5, It works correctly.

What did you expect to see?

If load an .so built with different verison of go，while error occur，I load another .so built with the correct version of go，no error should occur，It should work correctly。

What did you see instead?

[FiloSottile]

/cc @ianlancetaylor

[ianlancetaylor]

I guess that if loading the plugin fails, we could call dlclose. Seems low priority but I'd be willing to review a CL.