core: protect against modem removal in critical callbacks (rh #553953)

There are more places to handle, but these are the most critical.  If
the modem is removed while a command is in-progress, the mm-callback-info
code will set info->modem to NULL.  Make sure we check for that in
callbacks and return a reasonable error.  Previous code would just
blindly forge ahead and die on a null dereference.

src/mm-errors.h

@@ -38,7 +38,8 @@ enum {
     MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED = 1,
     MM_MODEM_ERROR_CONNECTED = 2,
     MM_MODEM_ERROR_DISCONNECTED = 3,
-    MM_MODEM_ERROR_OPERATION_IN_PROGRESS = 4
+    MM_MODEM_ERROR_OPERATION_IN_PROGRESS = 4,
+    MM_MODEM_ERROR_REMOVED = 5
 };
 
 #define MM_MODEM_ERROR (mm_modem_error_quark ())