Null termnate the pUserMappings variable to prevent memory corruption.

src/joystick/SDL_gamecontroller.c

@@ -835,6 +835,7 @@ SDL_GameControllerLoadHints()
         char *pUserMappings = SDL_malloc( nchHints + 1 );
         char *pTempMappings = pUserMappings;
         SDL_memcpy( pUserMappings, hint, nchHints );
+        pUserMappings[nchHints] = '\0';
         while ( pUserMappings ) {
             char *pchNewLine = NULL;