dpif-linux: Fix segfault when a port already exists.

dpif-linux: Fix segfault when a port already exists.

Commit 78a2d59c (dpif-linux.c: Let the kernel pick a port number if one
not requested.) changed the logic for port assignment, but didn't
properly handle some error conditions.  An attempt to add a tunnel port
that already exists would lead to a segfault.  This commit fixes the
logic to stop processing and return an error.

Reported-by: Gurucharan Shetty <shettyg@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>