bug in xmlRegExecPushString2() pointed out by Sreeni Nair. Daniel

* xmlregexp.c: bug in xmlRegExecPushString2() pointed out by
  Sreeni Nair.
Daniel
diff --git a/xmlregexp.c b/xmlregexp.c
index 97e9be7..55b0954 100644
--- a/xmlregexp.c
+++ b/xmlregexp.c
@@ -3679,7 +3679,7 @@
         ret = xmlRegExecPushStringInternal(exec, str, data, 1);
 
     if (str != buf)
-        xmlFree(buf);
+        xmlFree(str);
     return(ret);
 }