swscale: fix NULL checking in sws_alloc_context()

sws_getCachedContext() and sws_getContext() expect sws_alloc_context()
to return NULL when out of memory, as follows.

    if (!(context = sws_alloc_context()))
        return NULL;

This patch fixes sws_alloc_context() to return NULL in that case.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>