Check string passed to extrainfo_parse_entry_from_string

scan-build reports the following out of bounds access, because it doesn't know that s is always non-NULL. But we should guard against it being NULL.

extrainfo_parse_entry_from_string:

+  if (BUG(!s))
+    return;
   while (end > s+2 && *(end-1) == '\n' && *(end-2) == '\n')