avformat/dashdec: Fix crash on invalid input/ENOMEM, fix leak

In case a SupplementalProperty node exists in an adaptationset,
it is searched for a "schemeIdUri" property via xmlGetProp().
Whatever xmlGetProp() returns is then compared via av_strcasecmp()
to a string literal. xmlGetProp() can return NULL, namely in case
no "schemeIdUri" exists and (given that this string is allocated)
presumably also on allocation failure. No check for NULL is done,
so this may crash.

Furthermore, the string returned by xmlGetProp() needs to be freed
with xmlFree(), but this is not done either.

This commit fixes both of these issues; they existed since this code
has been added in 10d008f0fd9e713e290f626300d66382ad786c49.

This has been found while investigating ticket #9697. The continuous
leaks might very well be the reason behind the observed slowdown.

Reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
