[mle] increase Route TLV length to avoid buffer overflow (#2431)

Properly formed Route TLVs should never have more than 32 route entries.
This fix follows a "be liberal in what you accept" approach.

Credit to OSS-Fuzz.

src/core/thread/mle_tlvs.hpp

@@ -603,7 +603,7 @@ private:
     };
     uint8_t mRouterIdSequence;
     uint8_t mRouterIdMask[BitVectorBytes(kMaxRouterId + 1)];
-    uint8_t mRouteData[kMaxRouters];
+    uint8_t mRouteData[kMaxRouterId + 1];
 } OT_TOOL_PACKED_END;
 
 /**