vp9: Read the frame size as unsigned

Sample-Id: 00001723-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit 925c37874b617816b6f460c6f229c18b46548b46)
swscale: Fix an undefined behaviour

Prevent a division by zero down the codepath.

Sample-Id: 00001721-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit 3a177a9cca924e097265b32f9282814f6b653e08)
rtmpproto: Check the buffer sizes when copying app/playpath strings

As pointed out by Reimar Döffinger.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 0bacfa8d37710b904897e7cbeb8d6f96fbf75e2e)

Conflicts:
	libavformat/rtmpproto.c