unlzma: fix a case where we could read before beginning of buffer

Testcase:

  21 01 01 00 00 00 00 00 e7 01 01 01 ef 00 df b6
  00 17 02 10 11 0f ff 00 16 00 00

Unfortunately, the bug is not reliably causing a segfault,
the behavior depends on what's in memory before the buffer.

function                                             old     new   delta
unpack_lzma_stream                                  2762    2768      +6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

archival/libarchive/decompress_unlzma.c

@@ -290,8 +290,11 @@ unpack_lzma_stream(transformer_state_t *xstate)
 				uint32_t pos;
 
 				pos = buffer_pos - rep0;
-				if ((int32_t)pos < 0)
+				if ((int32_t)pos < 0) {
 					pos += header.dict_size;
+					if ((int32_t)pos < 0)
+						goto bad;
+				}
 				match_byte = buffer[pos];
 				do {
 					int bit;