vpxenc: open output file after setting pass #

write_ivf_file_header would incorrectly skip writing the file header in
the 2nd pass, causing the initial frame header to be overwritten on
close potential causing an overly large frame header to be read and a
crash.

most likely broken since:
9e50ed7f vpxenc: initial implementation of multistream support

fixes issue #585

Change-Id: I7e863e295dd6344c33b3e9c07f9f0394ec496e7b