Catch exceptions by reference

We use polymorphic exception objects, so catching by value invokes
the copy constructor and stuff that we don't really want.
diff --git a/common/rfb/HTTPServer.cxx b/common/rfb/HTTPServer.cxx
index 54becbb..2895a69 100644
--- a/common/rfb/HTTPServer.cxx
+++ b/common/rfb/HTTPServer.cxx
@@ -125,7 +125,7 @@
     while (1) {
       os.writeU8(is.readU8());
     }
-  } catch (rdr::EndOfStream) {
+  } catch (rdr::EndOfStream&) {
   }
 }