In g_newprovider_event() return if G_P_WITHER is set

[trasz]

Summary

In g_newprovider_event() return if G_P_WITHER is set.

This fixes a failed assertion in scenario where the provider
disappears, disk_gone() gets called, and at the exact same
time something else closes the device node triggering a retaste.

[mav]

Looks good. Few places that call it also check for G_PF_WITHER, but they are using g_post_event() for it, that opens a race window.

[trasz]

(Tinderboxed.)