Tests: Add fuzz_encode_stream ossfuzz target.

This fuzz target handles .xz stream encoding. The first byte of input
is used to dynamically set the preset level in order to increase the
fuzz coverage of complex critical code paths.

tests/ossfuzz/config/fuzz_encode_stream.options

@@ -0,0 +1,2 @@
+[libfuzzer]
+max_len = 4096

tests/ossfuzz/fuzz_encode_stream.c

@@ -0,0 +1,79 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+/// \file       fuzz_encode_stream.c
+/// \brief      Fuzz test program for liblzma lzma_stream_encoder() w/ LZMA2
+//
+//  Author:     Maksym Vatsyk
+//
+//  Based on Lasse Collin's original fuzzer for liblzma
+//
+//  This file has been put into the public domain.
+//  You can do whatever you want with this file.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#include <inttypes.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include "lzma.h"
+#include "fuzz_common.h"
+
+
+extern int
+LLVMFuzzerTestOneInput(const uint8_t *inbuf, size_t inbuf_size)
+{
+	if (inbuf_size == 0) {
+		fprintf(stderr, "no input data provided\n");
+		return 0;
+	}
+
+	// set LZMA preset level based on the first input byte
+	uint32_t preset_level;
+	uint8_t decider = inbuf[0];
+	switch (decider) {
+	case 0:
+	case 1:
+	case 5:
+		preset_level = (uint32_t)decider;
+		break;
+	case 6:
+		preset_level = 0 | LZMA_PRESET_EXTREME;
+		break;
+	case 7:
+		preset_level = 3 | LZMA_PRESET_EXTREME;
+		break;
+	default:
+		return 0;
+	}
+
+	// Initialize lzma_options with the above preset level
+	lzma_options_lzma opt_lzma;
+	if (lzma_lzma_preset(&opt_lzma, preset_level)){
+		fprintf(stderr, "lzma_lzma_preset() failed\n");
+		abort();
+	}
+
+	// Initialize filter chain for lzma_stream_decoder() call
+	// Use single LZMA2 filter for encoding
+	lzma_filter filters[2];
+	filters[0].id = LZMA_FILTER_LZMA2;
+	filters[0].options = &opt_lzma;
+	filters[1].id = LZMA_VLI_UNKNOWN;
+
+	// initialize empty LZMA stream
+	lzma_stream strm = LZMA_STREAM_INIT;
+
+	// Initialize the stream encoder using the above
+	// stream, filter chain and CRC64.
+	if (lzma_stream_encoder(&strm,
+			filters, LZMA_CHECK_CRC64) != LZMA_OK) {
+		fprintf(stderr, "lzma_stream_encoder() failed\n");
+		abort();
+	}
+
+	fuzz_code(&strm, inbuf  + 1, inbuf_size - 1);
+
+	// Free the allocated memory.
+	lzma_end(&strm);
+	return 0;
+}