[Downloader] Only attempt to copy the download if theres a source location

Closes https://github.com/CocoaPods/CocoaPods/issues/3433

CHANGELOG.md

@@ -4,6 +4,16 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
 
 To install release candidates run `[sudo] gem install cocoapods --pre`
 
+## Master
+
+##### Bug Fixes
+
+* Don't crash when the downloader can't find an appropriate podspec in a `git`
+  pod.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#3433](https://github.com/CocoaPods/CocoaPods/issues/3433)
+
+
 ## 0.37.0.beta.1
 
 ##### Enhancements

lib/cocoapods/downloader.rb

@@ -32,7 +32,7 @@ module Pod
       cache_path, tmp_cache = Pathname(Dir.mktmpdir), true unless cache_path
       cache = Cache.new(cache_path)
       result = cache.download_pod(request)
-      if target
+      if target && result.location
         UI.message "Copying #{request.name} from `#{result.location}` to #{UI.path target}", '> ' do
           FileUtils.rm_rf target
           FileUtils.cp_r(result.location, target)