op_stroff

Convert operand to an offset in a structure
(for the explanations of 'ea' and 'n' please see @hlpIdcop_bin[op_bin]())
     strid   - id of a structure type
     strpath - instead of strid, it is possible to specify a full strpath
               like this: "struct.mem1.mem2"
     delta - struct offset delta. usually 0. denotes the difference
             between the structure base and the pointer into the structure.

success op_stroff(long ea, int n, long strid, long delta);
success op_stroff(long ea, int n, string strpath, long delta);