filterANSIEscapes: Ignore BEL character

GCC is not as good at music as it seems to think it is.  Fixes #4546.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/libutil/util.cc

@@ -1412,7 +1412,7 @@ std::string filterANSIEscapes(const std::string & s, bool filterAll, unsigned in
             }
         }
 
-        else if (*i == '\r')
+        else if (*i == '\r' || *i == '\a')
             // do nothing for now
             i++;