avoid nil pointer panic (thread save)

[d-g-laptev]

Issue #43 didn't fix problem
I got panic:

	runtime error: invalid memory address or nil pointer dereference
	goroutine 19231 [running]:
	******
	panic(0x1c60160, 0x29801c0)
		/usr/local/Cellar/go/1.13.4/libexec/src/runtime/panic.go:679 +0x1b2
	github.com/antchfx/xpath.(*attributeQuery).Select(0xc0019bba20, 0x201ee00, 0xc0087c4a40, 0x2034f40, 0xc0019bba20)
		/Users/d.laptev/Dev/go/pkg/mod/github.com/antchfx/xpath@v1.1.8/query.go:147 +0x2f
	github.com/antchfx/xpath.normalizespaceFunc(0x2034f40, 0xc0019bba20, 0x201ee00, 0xc0087c4a40, 0xc0048fc350, 0xc0048fc350)
		/Users/d.laptev/Dev/go/pkg/mod/github.com/antchfx/xpath@v1.1.8/func.go:354 +0x444

Panic didn't happen when i disable cache in htmlquery

[coveralls]

Coverage increased (+0.1%) to 75.814% when pulling 44e7a6b on d-g-laptev:fix_panic into 51d8936 on antchfx:master.