Fix dangling pointer on temporary pack of player file on join (#706)

I hate StdStrBuf. It's just close enough to any sane string class that you think
it does reasonable things, then when you don't look it will turn around and stab
you in the back with a rusty fork.

src/control/C4Control.cpp

@@ -680,7 +680,7 @@ C4ControlJoinPlayer::C4ControlJoinPlayer(const char *szFilename, int32_t iAtClie
 			MakeTempFilename(&filename_buf);
 			if (C4Group_PackDirectoryTo(filename.getData(), filename_buf.getData()))
 			{
-				filename = filename_buf;
+				filename.Take(filename_buf);
 				file_is_temp = true;
 			}
 			else