Avoid signaling errors when computing "Stop" in GDB menu

* lisp/progmodes/gdb-mi.el (gdb-show-stop-p): Don't assume
'gdb-running-threads-count' must have a numeric value.
(Bug#24414)

lisp/progmodes/gdb-mi.el

@@ -1976,6 +1976,7 @@ is running."
             (not gdb-non-stop))
            gud-running)
       (and gdb-gud-control-all-threads
+           (not (null gdb-running-threads-count))
            (> gdb-running-threads-count 0))))
 
 ;; GUD displays the selected GDB frame.  This might might not be the current