Scripting: require at least one argument for redis.call().

Redis used to crash with a call like the following:

    EVAL "redis.call()" 0

Now the explicit check for at least one argument prevents the problem.

This commit fixes issue #655.

src/scripting.c

@@ -167,6 +167,13 @@ int luaRedisGenericCommand(lua_State *lua, int raise_error) {
     redisClient *c = server.lua_client;
     sds reply;
 
+    /* Require at least one argument */
+    if (argc == 0) {
+        luaPushError(lua,
+            "Please specify at least one argument for redis.call()");
+        return 1;
+    }
+
     /* Build the arguments vector */
     argv = zmalloc(sizeof(robj*)*argc);
     for (j = 0; j < argc; j++) {

tests/unit/scripting.tcl

@@ -145,6 +145,12 @@ start_server {tags {"scripting"}} {
         set e
     } {*not allowed after*}
 
+    test {EVAL - No arguments to redis.call/pcall is considered an error} {
+        set e {}
+        catch {r eval {return redis.call()} 0} e
+        set e
+    } {*one argument*}
+
     test {EVAL - redis.call variant raises a Lua error on Redis cmd error (1)} {
         set e {}
         catch {