encoder: fix null buffer dereferencing am: 3139d13ccd am: 3a4aac7376 am: 29b671c12b am: 528bff6aa4

Original change: https://android-review.googlesource.com/c/platform/external/libavc/+/1663964

Change-Id: Ib7c3b278ec885e3433a6ca3292381447582e7389