Var: improved variable parsing with empty names.

Unit parsed the case of "$uri$$host" into unknown variables.
This commit makes it invalid variable instead.

test/test_variables.py

@@ -378,6 +378,7 @@ Connection: close
         check_variables("${")
         check_variables("${}")
         check_variables("$ur")
+        check_variables("$uri$$host")
         check_variables("$uriblah")
         check_variables("${uri")
         check_variables("${{uri}")