(rasqal_new_values_graph_pattern): NULL pointer [coverity CID 43037]
(rasqal_raptor_init_triples_source_common): Error path [coverity CID 43038]

(rasqal_raptor_init_triples_source_common): Shortcut return path when
there are no source literals or error path [coverity CID 43038]
(rasqal_assignment_rowsource_read_row): NULL row [coverity CID 43039]
(rasqal_literal_set_typed_value): double/float always free string [coverity CID 43051]
Fix multiple checks where error_p may be NULL