Handle SID Index in SRGB block

Handle SID Index in SRGB block

When building Graph from BGP-LS, Segment Routing Global Block (SRGB)
advertized with a SID Index instead of an MPLS Label thrown a Java
Exception in getVertex() function. This commit allows the support of
both SID Index and MPLS Label for SRGB lower bound value.

JIRA: BGPCEP-970
Change-Id: I14cf39831f5ef47f7e80e91659ae6daeeacb1b91
Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 948399876d9b8b237f4597aee0e7d1d8b551793f)