Bug 564705. Don't try to insert a separating space at the start of the XUL textbox title. r=neil

layout/xul/base/src/nsTextBoxFrame.cpp

@@ -876,7 +876,7 @@ nsTextBoxFrame::UpdateAccessTitle()
     }
 
     if (InsertSeparatorBeforeAccessKey() &&
-        !NS_IS_SPACE(mTitle[offset - 1])) {
+        offset > 0 && !NS_IS_SPACE(mTitle[offset - 1])) {
         mTitle.Insert(' ', offset);
         offset++;
     }