gstbuffer: reset buffer to its original size if intact

gstbuffer: reset buffer to its original size if intact

Enhance default_reset_buffer() to resize the buffer to its full size if the
memory hasn't changed. This allows to reuse the buffer even if the offset has
changed or the size has shrunk, rather than freeing the buffer.

Change related to: https://bugzilla.gnome.org/show_bug.cgi?id=772841