fix warning: "Assigned value is garbage"

clang scan-build noticed that find_command() may bitmask `eap->flags`.
cmd_can_preview() only uses `ea.cmdidx`, but let's fix the warning...

Found by clang scan-build 5.0

src/nvim/ex_docmd.c

@@ -10048,6 +10048,7 @@ bool cmd_can_preview(char_u *cmd)
   }
 
   exarg_T ea;
+  memset(&ea, 0, sizeof(ea));
   // parse the command line
   ea.cmd = skip_range(cmd, NULL);
   if (*ea.cmd == '*') {