Song change causes crash (double free or corruption)

[l p]

When playing a game, bzr6539 crashed after first song has been played completely.

*** glibc detected *** ./widelands: double free or corruption (fasttop): 0x0000000004c9dda0 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x7eb96)[0x7fe40e882b96]
./widelands(_ZN7Songset8get_songEv+0x87)[0x64cfb7]
./widelands[0x647c32]
./widelands(_ZN13Sound_Handler12change_musicERKSsii+0xe2)[0x648932]
./widelands(_ZN13WLApplication10poll_eventER9SDL_Eventb+0x1ad)[0x518add]
./widelands(_ZN13WLApplication12handle_inputEPK13InputCallback+0x49)[0x518c39]
./widelands(_ZN2UI5Panel3runEv+0x17a)[0x6c32ea]
./widelands(_ZN9Widelands4Game3runEPN2UI14ProgressWindowENS0_15Start_Game_TypeE+0x410)[0x5fd3e0]
./widelands(_ZN13WLApplication8new_gameEv+0x466)[0x51af76]
./widelands(_ZN13WLApplication21mainmenu_singleplayerEv+0xba)[0x51c34a]
./widelands(_ZN13WLApplication8mainmenuEv+0x358)[0x51f868]
./widelands(_ZN13WLApplication3runEv+0x12b)[0x51fecb]
./widelands(main+0x99)[0x513a99]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed)[0x7fe40e82576d]
./widelands[0x5169b5]

Offending line would seem to be songset.cc:85; SDL_FreeRW(m_rwops) ?

This bug may be related to Bug #1150517.

Can provide complete stdout if needed.