It's valid to have a map entry that is null to represent a framebuffer
that has been generated but never bound.  This was causing crashes after
dEQP pbo tests were run during dEQP shutdown.

BUG=angleproject:1149

Change-Id: Idcedac23e5c4e17ed025b10c2939572b394d3aec
Reviewed-on: https://chromium-review.googlesource.com/296962Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>