(extracting_expect): Set $var to the empty string even on failure, to

	avoid some TCL errors.

src/server/testsuite/config/unix.exp

@@ -214,7 +214,9 @@ proc extracting_expect {regex var grp} {
         puts -nonewline "."
         flush stdout
     }
-        
+
+    set $var ""
+
     expect {
 	-re "^$line_leader$regex$nl" {
 	    set $var $expect_out($grp,string)