lavf/mov.c: Avoid OOB in mov_read_udta_string()

Core of patch is from paul@paulmehta.com
Reference https://crbug.com/643952 (udta_string portion)

Signed-off-by: Matt Wolenetz <wolenetz@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
