Import fixes from raptor

* const'ify string literals and char pointers that needn't be changed
  - librdfa (start_element): const'ify umap_key
  - librdfa (rdfa_complete_object_literal_triples): const'ifty
    current_object_literal
* (rdfa_init_base): Fix <base href >
* (end_element): Do not call rdfa_complete_list_triples() with no
  new subject [coverity]
* (rdfa_init_base): Fix uri_start dead code [coverity]
* (end_element): Add parent_context check around block [coverity]
* (rdfa_init_base): Check for uri_start before use [coverity]
* librdfa fixes for -Wunreachable-code [clang]
* (rdfa_complete_object_literal_triples) strchr on NULL [coverity]
* (rdfa_complete_object_literal_triples): Need non NULL
  context->xml_literal to do strchr() on it [coverity]
* (rdfa_complete_current_property_value_triples): Remove duplicate
  variable [coverity]
* (rdfa_complete_object_literal_triples): NULL check for
  context->xml_literal [coverity]