No check for failed popen at write_addrproto_config

**Describe the issue**
A static analysis tool flagged usage of fp pointer at /lib/rttables.c:621 as a potential issue, since popen may return NULL on failed memory allocation.

**To Reproduce**
Is a code-related issue found with SA tool.

**Expected behavior**
An additional NULL check after popen could prevent potential dereference.

**Keepalived version**
2.3.4
