Ruby: CFG: fix multiple successors failure for default parameters

Author: aibaars

ruby/ql/lib/codeql/ruby/controlflow/internal/ControlFlowGraphImpl.qll

@@ -923,7 +923,8 @@ module Trees {
         c instanceof SimpleCompletion
         or
         this.hasDefaultValue() and
-        c.(MatchingCompletion).getValue() = true
+        c.(MatchingCompletion).getValue() = true and
+        c.isValidFor(this)
       )
     }