pldmd: Account for pldm_entity_association_tree_init() returning NULL

As with handling pldm_pdr_init() failures, throw a std::runtime_error()
for pldm_entity_association_tree_init() failures. Throwing is an
improvement thanks to unwinding, by comparison to the immediate
termination invoked by assert().

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I3d5d6bed5afa76503f16ec9d1842cf4fd09e9ae7