Multiple reloads cause crash

If one manually reloads a file while Zathura is already reloading that file Zathura crashes with a segmentation fault.

Steps to reproduce:

• set filemonitor "noop" in the .rc (not sure if necessary)
• repeatedly trigger reloading a file really fast (alternatively trigger reloading on a huge file/page two times)

Zathura version: 0.4.5-1

Girara version: 0.3.4-1

zathura-pdf-mupdf version: 0.3.5-2

All three installed from the Arch Linux repositories.