journal-importer: avoid false maybe-uninitialized warning

journal-importer: avoid false maybe-uninitialized warning

Observerd on Ubuntu 24.04 with GCC 13 on arm64 architecture.
```
../src/shared/journal-importer.c: In function ‘journal_importer_process_data’:
../src/shared/journal-importer.c:344:30: error: ‘line’ may be used uninitialized [-Werror=maybe-uninitialized]
  344 |                         if (!journal_field_valid(line, n - 1, true)) {
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/journal-importer.c:295:23: note: ‘line’ was declared here
  295 |                 char *line, *sep;
      |                       ^~~~
cc1: all warnings being treated as errors
```