webm_info: fix clang -Wdangling-gsl warning

This warning detects when pointers are owned by an object, but that
object is deleted at the end of a statement.  Since the pointer is to
freed memory, accessing the pointer may produce unexpected results.

BUG=webm:1698

Change-Id: I200508e23b2bdafadbbfe535de0906739503f6ad