fix: panic if repeat defer mat.Close

[opuqo]

Simple code where problem was found:

import (
	"fmt"
	"gocv.io/x/gocv"
)

func main() {
	mat1 := gocv.NewMat()
	defer mat1.Close()
	mat2 := gocv.NewMat()
	mat1 = mat2
	defer mat2.Close()
}

If did't check on nil in func Close, get panic

[signal SIGSEGV: segmentation violation code=0x1 addr=0x7ff60000003c pc=0x4e3a79]

runtime stack:
runtime.throw(0x532266, 0x2a)
        /usr/local/go/src/runtime/panic.go:1116 +0x72
runtime.sigpanic()
        /usr/local/go/src/runtime/signal_unix.go:726 +0x4ac

goroutine 1 [syscall]:
runtime.cgocall(0x4d83a0, 0xc000050ea0, 0x0)
        /usr/local/go/src/runtime/cgocall.go:133 +0x5b fp=0xc000050e70 sp=0xc000050e38 pc=0x43b3bb
gocv.io/x/gocv._Cfunc_Mat_Close(0x7ff6f2380ce0)
        _cgo_gotypes.go:3080 +0x45 fp=0xc000050ea0 sp=0xc000050e70 pc=0x4d40c5```

[deadprogram]

Hello @opuqo thanks for the contribution.

I am just wondering is returning an error really needed? Or perhaps just return might be sufficient to avoid the issue. Mainly we have wanted to make sure it gets called at all. The CI tests are failing in your branch because this does apparently happen someplace related to the profiling code.

Also, please note I have changed the branch to dev as mentioned here: https://github.com/hybridgroup/gocv/blob/release/CONTRIBUTING.md#how-to-use-our-github-repository