Fix two UaF's where memory in `args` was accessed through `c` pointer
after freeing `args`, silencing the respective compiler warnings on
Linux/GCC.  The UaFs are on error paths as part of parsing the command
line, and as such unreachable from malicious response packets.

While here, remove dead code after `log_fatal()`, avoid interpreting
percent chars in `ipip_usage_error`, replace a manual if/exit combo with
an assertion, and remove extra newlines in log strings.