Moved cleanup to end of read_structure_array_with_fields function

* src/rmatio.c (read_structure_array_with_fields): Moved all cleanup of
  UNPROTECT to end of function
Moved cleanup to end of read_structure_array_with_empty_fields

* src/rmatio.c (read_structure_array_with_empty_fields): Moved cleanup
  of UNPROTECT to end of function
Moved cleanup to end of read_empty_structure_array_with_fields

* src/rmatio.c (read_empty_structure_array_with_fields): Moved all
  cleanup of UNPROTECT to end of function
Updated doxygen documentation in rmatio.c