bgpd: Fix session reset issue caused by malformed core attributes

bgpd: Fix session reset issue caused by malformed core attributes

RCA:
On encountering any attribute error for core attributes in update message,
the error handling is set to 'treat as withdraw' and
further parsing of the remaining attributes is skipped.
But the stream pointer is not being correctly adjusted to
point to the next NLRI field skipping the rest of the attributes.
This leads to incorrect parsing of the NLRI field,
which causes BGP session to reset.

Fix:
The stream pointer offset is rightly adjusted to point to the NLRI field correctly
when the malformed attribute is encountered and remaining attribute parsing is skipped.

Signed-off-by: Samanvitha B Bhargav <bsamanvitha@vmware.com>