Fix crash when retrieve subspec from a unavailable podspec file

[sagiwei]

When install a subspec such as AFNetworking/Reachability with no version range is specified:

pod 'AFNetworking/Reachability'

Cocoapods will crashed:

NoMethodError - undefined method `downcase' for nil:NilClass

Same issue mentioned here: CocoaPods/CocoaPods#11733

---

After some research, I found it caused by AFNetworking (= 1.1.1), this podspec.json has empty contents:

{
  "deprecated_in_favor_of": "Alamofire",
  "platforms": {
    "osx": null,
    "ios": null,
    "tvos": null,
    "watchos": null
  }
}

so I add some check code in subspec_by_name method, and now no more crash occurs.