Pagespeed lost img tag src value,when pagespeed is too busy to rewrite image

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What steps will reproduce the problem?
1.Use apache as forword proxy
2.set ModPagespeedImageMaxRewritesAtOnce      1
3.Test pagespeed's performance with ab or loadrunner until pagespeed is Too 
busy to rewrite image.
If you want to reappear the issue quickly, you can modify the source code as 
below and rebuild.
RewriteResult ImageRewriteFilter::RewriteLoadedResourceImpl(
      Context* rewrite_context, const ResourcePtr& input_resource,
      const OutputResourcePtr& result) {
         if (work_bound_->TryToWork()) {
         }
      }
TO:
RewriteResult ImageRewriteFilter::RewriteLoadedResourceImpl(
      Context* rewrite_context, const ResourcePtr& input_resource,
      const OutputResourcePtr& result) {
         if (0) {
         }
      }   

What is the expected output? What do you see instead?
The orginal info: 
<img width="320" height="143" 
src="http://r3.sinaimg.cn/2/2013/0524/a0/9/03424793/320x143x100x0x0x0.jpg">

Expected:
<img width="320" height="143" 
src="http://r3.sinaimg.cn/2/2013/0524/a0/9/03424793/320x143w320x143x100x0x0x0.jp
g.acce.ic.5df2CTUCSE.webp">

Instead:
<img width="320" height="143" src="">

What version of the product are you using (please check X-Mod-Pagespeed
header)?

1.1.23.2-0

On what operating system?
SUSE

Which version of Apache?
Server version: Apache/2.4.3 (Unix)

Which MPM?
Server MPM:     event
URL of broken page:
http://3g.sina.com.cn/

Original issue reported on code.google.com by yanzhenl...@gmail.com on 24 May 2013 at 9:09