parsers: better bounds checking in fm1readmarkers

Our Python already calls this with reasonable values consistently, but
my upcoming fuzzer is extremely quick to discover the lack of sanity
checking here.

Differential Revision: https://phab.mercurial-scm.org/D5464

mercurial/cext/parsers.c

@@ -572,6 +572,17 b' static PyObject *fm1readmarkers(PyObject'
 	                      &offset, &stop)) {
 		return NULL;
 	}
+	if (offset < 0) {
+		PyErr_SetString(PyExc_ValueError,
+		                "invalid negative offset in fm1readmarkers");
+		return NULL;
+	}
+	if (stop > datalen) {
+		PyErr_SetString(
+		    PyExc_ValueError,
+		    "stop longer than data length in fm1readmarkers");
+		return NULL;
+	}
 	dataend = data + datalen;
 	data += offset;
 	markers = PyList_New(0);