wslua_tvb: fix documentation for reported_length_remaining().

In the past, tvb_reported_length_remaining(), and thus
Tvb:reported_length_remaining(), may have returned -1 if the offset was
invalid.  That's no longer the case; the former returns 0, and, as the
latter just returns the former's return value, that's true of the latter
as well.

epan/wslua/wslua_tvb.c

@@ -157,7 +157,7 @@ WSLUA_METHOD Tvb_len(lua_State* L) {
 }
 
 WSLUA_METHOD Tvb_reported_length_remaining(lua_State* L) {
-    /* Obtain the reported (not captured) length of packet data to end of a <<lua_class_Tvb,`Tvb`>> or -1 if the
+    /* Obtain the reported (not captured) length of packet data to end of a <<lua_class_Tvb,`Tvb`>> or 0 if the
        offset is beyond the end of the <<lua_class_Tvb,`Tvb`>>. */
 #define Tvb_reported_length_remaining_OFFSET 2 /* offset */
     Tvb tvb = checkTvb(L,1);