storage/curl: move start call out of the constructor

This can cause request completion in the I/O thread before this
constructor returns, leaving the object in an abstract state, causing
a crash due to pure virtual method call.  We should not start the
request until this object is fully constructed.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/665

src/storage/plugins/CurlStorage.cxx

@@ -109,7 +109,9 @@ public:
 			     BIND_THIS_METHOD(OnDeferredStart)),
 		 request(curl, uri, *this) {
 		// TODO: use CurlInputStream's configuration
+	}
 
+	void DeferStart() noexcept {
 		/* start the transfer inside the IOThread */
 		defer_start.Schedule();
 	}
@@ -283,6 +285,7 @@ public:
 	}
 
 	using BlockingHttpRequest::GetEasy;
+	using BlockingHttpRequest::DeferStart;
 	using BlockingHttpRequest::Wait;
 
 protected:
@@ -430,6 +433,7 @@ public:
 	}
 
 	const StorageFileInfo &Perform() {
+		DeferStart();
 		Wait();
 		return info;
 	}
@@ -481,6 +485,7 @@ public:
 		 base_path(UriPathOrSlash(uri)) {}
 
 	std::unique_ptr<StorageDirectoryReader> Perform() {
+		DeferStart();
 		Wait();
 		return ToReader();
 	}