bgpd: fix heap buffer overflow in lcom -> str enc

bgpd: fix heap buffer overflow in lcom -> str enc

Spaces were not being accounted for in the heap buffer sizing, leading
to a heap buffer overflow when encoding large communities to their
string representations.

This patch also uses safer functions to do the encoding instead of
pointer math.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>