Handle NULL input to protover_compute_for_old_tor()

Fixes bug 24245; bugfix on 0.2.9.4-alpha. TROVE-2017-010.

changes/trove-2017-010

@@ -0,0 +1,6 @@
+  o Major bugfixes (security):
+    - Fix a denial-of-service issue where an attacker could crash
+      a directory authority using a malformed router descriptor.
+      Fixes bug 24245; bugfix on 0.2.9.4-alpha. Also tracked
+      as TROVE-2017-010 and CVE-2017-8820.
+

src/or/protover.c

@@ -694,6 +694,11 @@ protocol_list_contains(const smartlist_t *protos,
 const char *
 protover_compute_for_old_tor(const char *version)
 {
+  if (version == NULL) {
+    /* No known version; guess the oldest series that is still supported. */
+    version = "0.2.5.15";
+  }
+
   if (tor_version_as_new_as(version,
                             FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS)) {
     return "";