The svg renderer doesn't stop properly if there are <fo:external-graphic ...>
elements in the fo document. 
But FOP creates an svg document which is ok, but there is no information about
the image in it.
Rendering the same fo document to PDF works fine.

Here is a sample xsl:fo code:

.......
  <fo:page-sequence master-reference="simple">

    <fo:flow flow-name="xsl-region-body">


   <fo:external-graphic src="bild3.gif"/>
<fo:block>
        The Extensible Markup Language (XML) is a subset of SGML that is
completely described in this document. Its goal is to
        enable generic SGML to be served, received, and processed on the Web in
the way that is now possible with HTML. XML
        has been designed for ease of implementation and for interoperability
with both SGML and HTML.
      </fo:block>
..........


And that the svg document FOP produces:

<?xml version="1.0" encoding="ISO-8859-1"?>
<svg contentScriptType="text/ecmascript" width="595.0" zoomAndPan="magnify"
     contentStyleType="text/css" height="842.0"
     preserveAspectRatio="xMidYMid meet" xmlns="http://www.w3.org/2000/svg"
     version="1.0">
    <g id="Page-1" style="font-family:sanserif;font-size:12"
       transform="translate(0,0.0)">
        <rect width="595.0" x="0.0" height="842.0" y="0.0"
              style="fill:none;stroke:black"/>
        <text x="70.866" y="171.69598">
            The
        </text>
        <text x="99.466" y="171.69598">
            Extensible
        </text>
............