This fix was cherry-picked by Mozilla from upstream libevent-2.1 to its
bundled copy of libevent-2.0.21 in mozilla-esr45.
* gnu/packages/patches/libevent-2.0-evbuffer-add-use-last-with-datap.patch:
New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/libevent.scm (libevent-2.0)[source][patches]: Add it.
Mark H Weaver