Fix regression of :drop introduced in 1a91000

A single line was deleted from `ex_drop()` in 1a91000 when fixing clint
warnings causing the `:drop` command to not work correctly if the buffer
is not already open in a window.

Fixes #4981

src/nvim/ex_cmds2.c

@@ -3533,5 +3533,6 @@ void ex_drop(exarg_T   *eap)
     } else {
       eap->cmdidx = CMD_first;
     }
+    ex_rewind(eap);
   }
 }