CDR: Protect from data overflow in ast_cdr_setuserfield.

ast_cdr_setuserfield wrote to a fixed length field using strcpy. This could
result in a buffer overrun when called from chan_sip or func_cdr. This patch
adds a maximum bytes written to the field by using ast_copy_string instead.

ASTERISK-26897 #close
patches:
  0001-CDR-Protect-from-data-overflow-in-ast_cdr_setuserfie.patch submitted
    by Corey Farrell (license #5909)

Change-Id: Ib23ca77e9b9e2803a450e1206af45df2d2fdf65c