parsers: fix 'unsigned expression is always true' warning (issue4142)

On Mac OS gcc-llvm throws an -Wtautological-compare warning because flen
is defined as an unsigned integer, therefore flen < 0 is always true.

mercurial/parsers.c

@@ -185,7 +185,7 b' static PyObject *parse_dirstate(PyObject'
 		flen = getbe32(cur + 13);
 		pos += 17;
 		cur += 17;
-		if (flen > len - pos || flen < 0) {
+		if (flen > len - pos) {
 			PyErr_SetString(PyExc_ValueError, "overflow in dirstate");
 			goto quit;
 		}