converting from v3 to v2 with an endpoint using a parameter with a schema ref used to fail with the following error:

```
--- FAIL: TestConvOpenAPIV3ToV2 (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x58 pc=0x100f71688]

goroutine 4 [running]:
testing.tRunner.func1.2({0x101035da0, 0x1012b6160})
	/opt/homebrew/Cellar/go/1.17/libexec/src/testing/testing.go:1209 +0x258
testing.tRunner.func1(0x14000117380)
	/opt/homebrew/Cellar/go/1.17/libexec/src/testing/testing.go:1212 +0x284
panic({0x101035da0, 0x1012b6160})
	/opt/homebrew/Cellar/go/1.17/libexec/src/runtime/panic.go:1038 +0x21c
github.com/getkin/kin-openapi/openapi2conv.FromV3Parameter(0x140001ce7c8, 0x1400007da18)
	/Users/vbehar/projects/oss/kin-openapi/openapi2conv/openapi2_conv.go:979 +0x138
github.com/getkin/kin-openapi/openapi2conv.FromV3Operation(0x1400007da00, 0x14000129c20)
	/Users/vbehar/projects/oss/kin-openapi/openapi2conv/openapi2_conv.go:895 +0x250
github.com/getkin/kin-openapi/openapi2conv.FromV3(0x1400007da00)
	/Users/vbehar/projects/oss/kin-openapi/openapi2conv/openapi2_conv.go:586 +0xddc
github.com/getkin/kin-openapi/openapi2conv.TestConvOpenAPIV3ToV2(0x14000117380)
	/Users/vbehar/projects/oss/kin-openapi/openapi2conv/openapi2_conv_test.go:26 +0x19c
testing.tRunner(0x14000117380, 0x1010870e8)
	/opt/homebrew/Cellar/go/1.17/libexec/src/testing/testing.go:1259 +0x104
created by testing.(*T).Run
	/opt/homebrew/Cellar/go/1.17/libexec/src/testing/testing.go:1306 +0x328
FAIL	github.com/getkin/kin-openapi/openapi2conv	0.840s
FAIL
```

This change fixes the issue by ensuring that we correctly handle a schema ref.

Signed-off-by: Pierre Fenoll <[email protected]>

Signed-off-by: Pierre Fenoll <[email protected]>

Signed-off-by: Pierre Fenoll <[email protected]>