qed: don't pass NULL to memcpy

Spotted by Clang Analyzer

[Note this memcpy call has always been safe because the length will be 0
when the pointer is NULL]

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/qed.c

diff --git a/block/qed.c b/block/qed.c
index e87dc4d..7fab155 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -1425,8 +1425,10 @@
     memcpy(buffer, &le_header, sizeof(le_header));
     buffer_len = sizeof(le_header);
 
-    memcpy(buffer + buffer_len, backing_file, backing_file_len);
-    buffer_len += backing_file_len;
+    if (backing_file) {
+        memcpy(buffer + buffer_len, backing_file, backing_file_len);
+        buffer_len += backing_file_len;
+    }
 
     /* Write new header */
     ret = bdrv_pwrite_sync(bs->file, 0, buffer, buffer_len);