flymake-diag-region really returns nil if region is invalid (bug#29174)

Reported by Lele Gaifax <lele@metapensiero.it>.

* lisp/progmodes/flymake.el (flymake-diag-region): Really
return nil if the region is invalid.

lisp/progmodes/flymake.el

@@ -343,7 +343,8 @@ region is invalid."
                 (let* ((beg (fallback-bol))
                        (end (fallback-eol beg)))
                   (cons beg end)))))))
-    (error (flymake-log :warning "Invalid region line=%s col=%s" line col))))
+    (error (flymake-log :warning "Invalid region line=%s col=%s")
+           nil)))
 
 (defvar flymake-diagnostic-functions nil
   "Special hook of Flymake backends that check a buffer.