Fix an incorrect call to ast_string_field_free_memory which caused a crash in the logger.

Since the message structure is allocated using ast_calloc_with_stringfields we do not need to
free the memory used for the stringfields as it will get freed when the message structure is.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@197570 65c4cc65-6c06-0410-ace0-fbb531ad65f3