mkrelease: Remove zip file before recreating it

Don't want to update an existing zip, always want a fresh one.

mkrelease.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/sh -e
 
 # This script is intended to run from cygwin or native Unix to ensure that the
 # symbolically linked DLLs are correctly resolved in the zip file.
@@ -11,4 +11,6 @@ if [ ! -d "$dir" ]; then
 	exit 1
 fi
 
+rm -fv "${game}.zip" || true
+
 cd "$dir" && zip -9 -r "../${game}.zip" .