We accomplish this by explicitly setting the global_obs to NULL after
free'ing it. The crash that led to this fix can be replicated as such
$ sendosc localhost 3819 /set_surface i 8 i 159 i 8
$ sendosc localhost 3819 /set_surface i 0 i 0 i 0
$ sendosc localhost 3819 /set_surface i 0 i 0 i 0
In this example the observer is created, then free'd, and then free'd
*again* because the new observer was never made (sur->feedback[x]
checks fail)
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