[Bug-gnuzilla] 100% cpu usage when using http proxy under certain circumstances

[Li Er]

actual behavior:
  when icecat is using an http proxy to connect to https sites, if the proxy 
connection died when icecat is doing tls handshake with the https server, the 
cpu usage of icecat jumps to 100% and the status bar still says "Waiting for 
...".

expected behavior:
  icecat should detect that the proxy server closed the connection and display 
appropriate message, also the cpu usage should be normal.

reproduce:
  create a dummy http proxy by netcat:
    nc -l -s 127.0.0.1 -p 8080

  configure icecat to use http proxy 127.0.0.1:8080 and check "Use this proxy 
server for all protocols".

  type https://en.wikipedia.org at the address bar, now the `nc' command should 
ouput something like:

CONNECT en.wikipedia.org:443 HTTP/1.1
Proxy-Connection: keep-alive
Connection: keep-alive
Host: en.wikipedia.org:443

  now feed "HTTP/1.1 200 OK\n\n" to the stdin of `nc', and the tls ClientHello 
message should be seen, then type control-c to terminate `nc'. the cpu usage of 
icecat jumps to 100%.

additional info:
  i'm using icecat version "GNU IceCat 38.5.2", compiled from source to x86-64 
architecture using gcc-4.8.5, binutils-2.25.1.
  configured using --disable-dbus --disable-pulseaudio --disable-gstreamer 
--disable-necko-wifi.