opcode_read: Parse buffer size check was wrong.

According to the [z-machine spec](https://inform-fiction.org/zmachine/standards/z1point1/sect15.html#sread) ...

> (Interpreters are asked to halt with a suitable error message if the text
> or parse buffers have length of less than 3 or 6 bytes, respectively: this
> sometimes occurs due to a previous array being overrun, causing bugs which
> are very difficult to find.)

My check is incorrect here; I have no idea where I got 4 in any case from
this text, but also, this number measures _tokens_, not bytes...the buffer
size is actually `2 + 4 * parselen`, so anything larger than zero will be
greater than 6 bytes.

This fixes Moonmist exploding near the start, when it asks you to enter your
name.

Fixes #24.

mojozork.c

@@ -1333,7 +1333,7 @@ static void opcode_read(void)
     const uint8 parselen = *(parse++);
 
     dbg("max parse: %u\n", (unsigned int) parselen);
-    if (parselen < 4)
+    if (parselen == 0)
         GState->die("parse buffer is too small for reading");  // happens on buffer overflow.
 
     updateStatusBar();