nbd/server: Silence gcc false positive

The code has a while() loop that always initialized 'end', and
the loop always executes at least once (as evidenced by the assert()
just prior to the loop).  But some versions of gcc still complain
that 'end' is used uninitialized, so silence them.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180622125814.345274-1-eblake@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

nbd/server.c

@@ -1937,7 +1937,7 @@ static unsigned int bitmap_to_extents(BdrvDirtyBitmap *bitmap, uint64_t offset,
                                       unsigned int nb_extents,
                                       bool dont_fragment)
 {
-    uint64_t begin = offset, end;
+    uint64_t begin = offset, end = offset;
     uint64_t overall_end = offset + *length;
     unsigned int i = 0;
     BdrvDirtyBitmapIter *it;
@@ -1977,6 +1977,7 @@ static unsigned int bitmap_to_extents(BdrvDirtyBitmap *bitmap, uint64_t offset,
 
     bdrv_dirty_bitmap_unlock(bitmap);
 
+    assert(offset > end);
     *length = end - offset;
     return i;
 }