Fix memory leak when pasting text

According to SDL documentation[1], the returned pointer needs to be
freed. A glance at the source code confirms that the function allocates,
and also Valgrind complains about it.

Also if it couldn't allocate, the game no longer segfaults (std::strings
do not check if the pointer is non-NULL for operator+=).

[1]: https://wiki.libsdl.org/SDL_GetClipboardText

desktop_version/src/KeyPoll.cpp

@@ -141,7 +141,12 @@ void KeyPoll::Poll(void)
 				else if (	evt.key.keysym.sym == SDLK_v &&
 						keymap[SDLK_LCTRL]	)
 				{
-					keybuffer += SDL_GetClipboardText();
+					char* text = SDL_GetClipboardText();
+					if (text != NULL)
+					{
+						keybuffer += text;
+						SDL_free(text);
+					}
 				}
 			}
 			break;