parsers: drop spurious check of readlen value

We're about to check if len < 40 after assigning readlen to len, which
means that if len < 40 we'll still abort, but I'm about to add a
sensible exception to that failure, so let's just discard this useless
check.

mercurial/parsers.c

@@ -478,9 +478,6 b' static PyObject *parse_dirstate(PyObject'
 			      &str, &readlen))
 		goto quit;
 
-	if (readlen < 0)
-		goto quit;
-
 	len = readlen;
 
 	/* read parents */