Indra segfaults Rack at close

[davephillips]

Fedora 23 Linux
Rack 0.6.2c from source

Thanks for the very cool Indra, it's sounding great in my patches. Alas, it also appears to cause a problem that occurs when I close Rack. Here's what's reported when I quit Rack:

*** Error in `./Rack': free(): invalid size: 0x0000000001ee8160 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x77de5)[0x7f89edfb3de5]
/lib64/libc.so.6(+0x8022a)[0x7f89edfbc22a]
/lib64/libc.so.6(cfree+0x4c)[0x7f89edfbf78c]
./plugins/qwelk/plugin.so(_ZN11ModuleIndraD0Ev+0x3c)[0x7f89d656a67c]
./Rack(_ZN4rack12ModuleWidgetD2Ev+0x15d)[0x585cfd]
./plugins/qwelk/plugin.so(_ZN11WidgetIndraD0Ev+0x2c)[0x7f89d656a48c]
./Rack(_ZN4rack6WidgetD1Ev+0x82)[0x59d992]
./Rack(_ZN4rack15ModuleContainerD0Ev+0x10)[0x592b80]
./Rack(_ZN4rack6WidgetD1Ev+0x82)[0x59d992]
./Rack(_ZN4rack10RackWidgetD0Ev+0x36)[0x58de76]
./Rack(_ZN4rack6WidgetD1Ev+0x82)[0x59d992]
./Rack(_ZN4rack10ZoomWidgetD0Ev+0x15)[0x59bfd5]
./Rack(_ZN4rack6WidgetD1Ev+0x82)[0x59d992]
./Rack(_ZN4rack6WidgetD0Ev+0x9)[0x59da59]
./Rack(_ZN4rack6WidgetD1Ev+0x82)[0x59d992]
./Rack(_ZN4rack16RackScrollWidgetD0Ev+0x15)[0x594185]
./Rack(_ZN4rack6WidgetD1Ev+0x82)[0x59d992]
./Rack(_ZN4rack9RackSceneD0Ev+0x15)[0x574595]
./Rack(main+0x55b)[0x4cde7b]
/lib64/libc.so.6(__libc_start_main+0xf0)[0x7f89edf5c580]
./Rack(_start+0x29)[0x4d1d99]

A long memory map follows. I've posted the full report to https://pastebin.com/b5FmtB9w