dlltool invalid free

This is a followup to commt 619f863c55ca "dlltool memory leaks".
The name passed to def_name is freed, so if missing we can't just
use "".  strdup it.

	* defparse.y (opt_name): xstrdup empty string.

binutils/defparse.y

diff --git a/binutils/defparse.y b/binutils/defparse.y
index d50739e..a2c4c4b 100644
--- a/binutils/defparse.y
+++ b/binutils/defparse.y
@@ -213,7 +213,7 @@
 	  }
 	;
 opt_name: opt_name2 { $$ =$1; }
-	|		{ $$=""; }
+	|		{ $$ = xstrdup (""); }
 	;
 
 opt_ordinal: