out_es: Fix buffer size when converted_size is equal to 0.

[GuillaumeSmaha]

In some case, ES_BULK_CHUNK is not enough to increase size of thebuffer

Fix #4412

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

I didn't find a way to add a test because it seems already cover by the div0 test because the buffer size is 8000 which is superior to chunk size #define ES_BULK_CHUNK 4096

fluent-bit/tests/runtime/out_elasticsearch.c

Lines 368 to 421 in f207f44

	/* https://github.com/fluent/fluent-bit/issues/3905 */
	void flb_test_div0()
	{
	int ret;
	char record[8000];
	char record_header[] = "[1448403340,{\"key\":\"";
	flb_ctx_t *ctx;
	int in_ffd;
	int out_ffd;
	int i;
	/* Create context, flush every second (some checks omitted here) */
	ctx = flb_create();
	flb_service_set(ctx, "flush", "1", "grace", "1", NULL);
	/* Lib input mode */
	in_ffd = flb_input(ctx, (char *) "lib", NULL);
	flb_input_set(ctx, in_ffd, "tag", "test", NULL);
	/* Elasticsearch output */
	out_ffd = flb_output(ctx, (char *) "es", NULL);
	flb_output_set(ctx, out_ffd,
	"match", "test",
	NULL);
	/* Override defaults of index and type */
	flb_output_set(ctx, out_ffd,
	NULL);
	/* Enable test mode */
	ret = flb_output_set_test(ctx, out_ffd, "formatter",
	cb_check_nothing,
	NULL, NULL);
	/* Start */
	ret = flb_start(ctx);
	TEST_CHECK(ret == 0);
	/* create json */
	strncpy(&record[0], &record_header[0], strlen(record_header));
	for(i=strlen(record_header); i<sizeof(record)-4; i++) {
	record[i] = 'a';
	}
	record[sizeof(record)-4] = '"';
	record[sizeof(record)-3] = '}';
	record[sizeof(record)-2] = ']';
	record[sizeof(record)-1] = '\0';
	/* Ingest data sample */
	flb_lib_push(ctx, in_ffd, (char *) &record[0], strlen(record));
	sleep(2);
	flb_stop(ctx);
	flb_destroy(ctx);
	}

[GuillaumeSmaha]

@nokute78 I am made the PR as you requested

[nokute78]

@GuillaumeSmaha Thank you !