CVE-2015-8035 Fix XZ compression support loop

For https://bugzilla.gnome.org/show_bug.cgi?id=757466
DoS when parsing specially crafted XML document if XZ support
is compiled in (which wasn't the case for 2.9.2 and master since
Nov 2013, fixed in next commit !)
diff --git a/xzlib.c b/xzlib.c
index 0dcb9f4..1fab546 100644
--- a/xzlib.c
+++ b/xzlib.c
@@ -581,6 +581,10 @@
             xz_error(state, LZMA_DATA_ERROR, "compressed data error");
             return -1;
         }
+        if (ret == LZMA_PROG_ERROR) {
+            xz_error(state, LZMA_PROG_ERROR, "compression error");
+            return -1;
+        }
     } while (strm->avail_out && ret != LZMA_STREAM_END);
 
     /* update available output and crc check value */