Fix unused return value error from e4836f3c

Re-introducing the fgets() call in e4836f3c that has been removed in
bf55c30 triggers the "ignoring return value of fgets, declared with
attribute warn_unused_result" error. The even better way is to actually
use the return value and check it for non-NULL because only then
we did a successful fgets call where we can use its result.

gnucash/import-export/log-replay/gnc-log-replay.c

@@ -639,9 +639,9 @@ void gnc_file_log_replay (GtkWindow *parent)
                     {
                         do
                         {
-                            fgets(read_buf, sizeof(read_buf), log_file);
+                            read_retval = fgets(read_buf, sizeof(read_buf), log_file);
                             /*DEBUG("Chunk read: %s",read_retval);*/
-                            if (strncmp(record_start_str, read_buf, strlen(record_start_str)) == 0) /* If a record started */
+                            if (read_retval && strncmp(record_start_str, read_buf, strlen(record_start_str)) == 0) /* If a record started */
                             {
                                 process_trans_record(log_file);
                             }