Reject packet with malformed range field instead of raising assert.
This fixes CVE-2020-36420 and makes polipo conformant to RFC7233.
--- a/server.c	2014-05-15 02:19:43.000000000 +0400
+++ b/server.c	2021-08-11 13:42:55.156568391 +0300
@@ -1468,7 +1468,6 @@
     int port;
     int x, y, z;
 
-    assert(from >= 0 && (to < 0 || to > from));
     assert(closure == NULL);
     assert(!(object->flags & OBJECT_LOCAL));
     assert(object->type == OBJECT_HTTP);
@@ -1499,6 +1498,13 @@
         notifyObject(object);
         return 1;
     }
+
+    if(!(from >= 0 && (to < 0 || to > from))) {
+        do_log(L_ERROR, "Invalid Range field in the header: from=%d, to=%d.\n", from, to);
+        abortObject(object, 400, internAtom("Invalid range header field"));
+        notifyObject(object);
+        return 1;
+    }
 
     memcpy(name, ((char*)object->key) + x, y - x);
     name[y - x] = '\0';