journal: avoid undefined behaviour in float division by 0.0

Coverity says that's undefined. I'm pretty sure we always would get a nan, but
let's avoid (formally) undefined behaviour since that can cause compilers to do
strange things.

src/journal/compress.c

@@ -667,7 +667,7 @@ int decompress_stream_lz4(int in, int out, uint64_t max_bytes) {
 
         log_debug("LZ4 decompression finished (%zu -> %zu bytes, %.1f%%)",
                   total_in, total_out,
-                  (double) total_out / total_in * 100);
+                  total_in > 0 ? (double) total_out / total_in * 100 : 0.0);
  cleanup:
         munmap(src, st.st_size);
         return r;