Fix thumbnails sometimes not being resized in the save browser

This was because ImageRequest::Finish used its Width and Height members after calling Request::Finish,
after which the ImageRequest object may get deleted at any time by RequestManager. One solution to this
is to copy (or preferably move) important members to local variables in the Finish functions of
objects derived from Request and use only those after calling Request::Finish (or anything that
in turn calls that).

src/client/http/APIRequest.cpp

@@ -20,6 +20,9 @@ namespace http
 		try
 		{
 			ByteString data = Request::Finish(&result.status);
+			// Note that at this point it's not safe to use any member of the
+			// APIRequest object as Request::Finish signals RequestManager
+			// to delete it.
 			Client::Ref().ParseServerReturn(data, result.status, true);
 			if (result.status == 200 && data.size())
 			{

src/client/http/GetUserInfoRequest.cpp

@@ -15,6 +15,9 @@ namespace http
 	{
 		std::unique_ptr<UserInfo> user_info;
 		auto result = APIRequest::Finish();
+		// Note that at this point it's not safe to use any member of the
+		// GetUserInfoRequest object as Request::Finish signals RequestManager
+		// to delete it.
 		if (result.document)
 		{
 			auto &user = (*result.document)["User"];

src/client/http/ImageRequest.cpp

@@ -19,7 +19,12 @@ namespace http
 
 	std::unique_ptr<VideoBuffer> ImageRequest::Finish()
 	{
+		int width = Width;
+		int height = Height;
 		ByteString data = Request::Finish(nullptr);
+		// Note that at this point it's not safe to use any member of the
+		// ImageRequest object as Request::Finish signals RequestManager
+		// to delete it.
 		std::unique_ptr<VideoBuffer> vb;
 		if (data.size())
 		{
@@ -35,7 +40,7 @@ namespace http
 				vb = std::unique_ptr<VideoBuffer>(new VideoBuffer(32, 32));
 				vb->SetCharacter(14, 14, 'x', 255, 255, 255, 255);
 			}
-			vb->Resize(Width, Height, true);
+			vb->Resize(width, height, true);
 		}
 		return vb;
 	}

src/client/http/SaveUserInfoRequest.cpp

@@ -18,6 +18,9 @@ namespace http
 	bool SaveUserInfoRequest::Finish()
 	{
 		auto result = APIRequest::Finish();
+		// Note that at this point it's not safe to use any member of the
+		// SaveUserInfoRequest object as Request::Finish signals RequestManager
+		// to delete it.
 		if (result.document)
 		{
 			return (*result.document)["Status"].asInt() == 1;