Fix warnings: ex_docmd.c: ex_tabonly(): Np dereference: FP.

Problem    : Dereference of null pointer @ 5816.
Diagnostic : False positive.
Rationale  : first_tabpage is assumed to be NULL after calling
             goto_tabpage(), which should not be possible (first_tabpage
             should be not NULL before calling it, and only changed to
             another valid tab page).
Resolution : Assert first_tabpage after calling goto_tabpage().

Helped-by: oni-link <knil.ino@gmail.com>

src/nvim/ex_docmd.c

@@ -5813,6 +5813,7 @@ static void ex_tabonly(exarg_T *eap)
           break;
         }
       }
+      assert(first_tabpage);
       if (first_tabpage->tp_next == NULL) {
         break;
       }