Decoder: Fixed error handling for dangling fields

In case of dangling fields with gaps in frames enabled,
field pic in cur_slice was wrongly set to 0.

This would cause dangling field to be concealed as a frame, which would
result in a number of MB mismatch and hence a hang.

Bug: 34097672
Change-Id: Ia9b7f72c4676188c45790b2dfbb4fe2c2d2c01f8
(cherry picked from commit 1a13168ca3510ba91274d10fdee46b3642cc9554)