test-compress-benchmark: skip loop iteration if size is 0

Otherwise we would hit an assert in the compression code.

src/journal/test-compress-benchmark.c

@@ -105,6 +105,8 @@ static void test_compress_decompress(const char* label, const char* type,
                 int r;
 
                 size = permute(i);
+                if (size == 0)
+                        continue;
 
                 log_debug("%s %zu %zu", type, i, size);