An icon from gnome-themes-extras-0.9.0.tar.bz2 (specifically Nuvola/icons/scalable/emblems/emblem-important.svg) by David Vignoni.
Edited to look more like the PNG file by Bastique.
This library is free; permission is granted to copy, distribute and/or modify this library under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.