h264: prevent rbsp_buffer values from becoming duplicated

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h264: Zero rbsp_buffer earler to ensure no duplicated pointers can leak

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h264: Free rbsp_buffer before copying context over it

Fixes memleak
Fixes Ticket1900

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>