AVIWriter: Don't overflow the fourcc storage

Prevent user input from blowing past the fourcc storage on the stack.

Change-Id: I7bcb92257f082bae8611a65b8f018ea9477c4dbe
AVIWriter: Fix a comment typo

Change-Id: I760f95e9374d00f619d273e515a7bc62b115c5b4
Merge topic 'prevent-buffer-overflow' into master

d7ec6c1b AVIWriter: Fix a comment typo
3e7585ca AVIWriter: Don't overflow the fourcc storage