Heap-buffer-overflow write in FFmpeg MDASH resolve_content_path

When calculating the content path in handling of MPEG-DASH manifests, there's an out-of-bounds NUL-byte write one byte past the end of the buffer.When we call xmlNodeGetContent below [0], it returns a buffer precisely allocated to match the string length, using strdup internally. If this buffer is not an empty string, it is assigned to root_url at [1].If the last (non-NUL) byte in this buffer is not '/' then we append '/' in-place at [2]. This will write two bytes into the buffer, starting at the last valid byte in the buffer, writing the NUL byte beyond the end of the allocated buffer.
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