Fix nil pointer dereference

[waybackarchiver]

# ./annie -v

annie: version 0.10.3, A fast, simple and clean video downloader.

# ./annie https://example.com
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x8b8265]

goroutine 1 [running]:
github.com/iawia002/annie/extractors.Extract(0x7ffeeb72af43, 0x13, 0x0, 0x0, 0x0, 0x1, 0x0, 0xa, 0x0, 0x0, ...)
	/Users/mac/iawia002/go/src/github.com/iawia002/annie/extractors/extractors.go:104 +0x2f5
main.download(0x7ffeeb72af43, 0x13, 0x1, 0x1)
	/Users/mac/iawia002/go/src/github.com/iawia002/annie/main.go:131 +0x18d
main.main()
	/Users/mac/iawia002/go/src/github.com/iawia002/annie/main.go:253 +0x26d

Related issues:

• bug？不能解析与直接下载 #852
• panic: runtime error: invalid memory address or nil pointer dereference #814
• 部分bilibili视频解析不了 #794
• segmentation violation while downloading a PDF file #760
• 输入含protocol的不支持地址会出invalid memory address or nil pointer dereference error而非显示“不支持”信息 #754

fix #852
fix #814
fix #794
fix #760
fix #754

[iawia002]

LGTM, thanks!