LU-443 LNet: Only squawk when md->start is NULL on non-zero length

LU-443 LNet: Only squawk when md->start is NULL on non-zero length

Only squawk when md->start is NULL on non-zero length.
The md->start == NULL check prevents anyone from creating a ME/MD
with no buffer. These are used as backstop buffers to generate events
when traffic has exceeded the local buffer space.

Change-Id: I1389b0a45d3f8ff548f6400c66b30a69bafb4f39
Signed-off-by: Wally Wang <wang@cray.com>
Reviewed-on: http://review.whamcloud.com/989
Tested-by: Hudson
Reviewed-by: Cory Spitz <spitzcor@cray.com>
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>