mb2q: Make cover letter replies work again

Using a stale variable for comparison instead of the actual iterator does
not work well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/mb2q b/mb2q
index 1adbad6..0cd916a 100755
--- a/mb2q
+++ b/mb2q
@@ -500,7 +500,7 @@
         # Update patches which reference a cover letter
         for c in cover:
             for p in self.patches:
-                if o.msgid == p.inreplyto or o.msgid in p.references:
+                if c.msgid == p.inreplyto or c.msgid in p.references:
                     p.update_tags(c)
 
         # Sanitize Ccs: