Panic when querying malformed xpath

[WithoutPants]

Added the following to TestXPath in query_test.go:

_, err := QueryAll(testDoc, "$test/@attr")

I'd expect this to return an error during parsing due to the badly formed xpath. Instead, it causes a panic:

--- FAIL: TestXPath (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x18 pc=0x543fd5]

goroutine 15 [running]:
testing.tRunner.func1.1(0x7423c0, 0xaee130)
        c:/go/src/testing/testing.go:940 +0x2fc
testing.tRunner.func1(0xc000111200)
        c:/go/src/testing/testing.go:943 +0x400
panic(0x7423c0, 0xaee130)
        c:/go/src/runtime/panic.go:969 +0x174
github.com/antchfx/xpath.(*attributeQuery).Clone(0xc0000054c0, 0xc000005520, 0x728220)
        C:/Users/.../go/pkg/mod/github.com/antchfx/[email protected]/query.go:164 +0x35
github.com/antchfx/xpath.(*Expr).Select(...)
        C:/Users/.../go/pkg/mod/github.com/antchfx/[email protected]/xpath.go:131
github.com/antchfx/htmlquery.QuerySelectorAll(0xc00010a230, 0xc0000054e0, 0xc0000054e0, 0x0, 0x0)
        D:/dev/htmlquery/query.go:82 +0x91
github.com/antchfx/htmlquery.QueryAll(0xc00010a230, 0x7ad043, 0xb, 0xc000191500, 0x0, 0x0, 0xa2b565, 0x1c)
        D:/dev/htmlquery/query.go:54 +0x8c
github.com/antchfx/htmlquery.TestXPath(0xc000111200)
        D:/dev/htmlquery/query_test.go:150 +0x2c1
testing.tRunner(0xc000111200, 0x7cbd00)
        c:/go/src/testing/testing.go:991 +0xe3
created by testing.(*T).Run
        c:/go/src/testing/testing.go:1042 +0x35e
exit status 2
FAIL    github.com/antchfx/htmlquery    0.270s