[3.0] contrib: ebml: use backported versions of string reading

The code is cleaner and doesn't leak local buffers when readFully() emits an exception. As in "oss-fuzz 5479158623043584".

Backport sent upstream at https://github.com/Matroska-Org/libebml/pull/323

(cherry picked from commit bee5a09d)

Backport of !7497 (merged)