This PR does some same refactoring of functions in strings.c to remove calls to STRLEN().

Specifically:
In vim_strsave_shellescape() a small cosmetic change.
In string_count() move the call to STRLEN() outside the while loop.
In copy_first_char_to_tv() change to return -1 on failure or the length of resulting v_string. Change string_filter_map() and string_reduce() to use the return value of copy_first_char_to_tv().
In blob_from_string() refactor to remove call to STRLEN().
In string_from_blob() call vim_strnsave() instead of vim_strsave().
In vim_snprintf_safelen() call vim_vsnprintf_typval() directly instead of vim_vsnprintf() which then calls vim_vsnprintf_typval().

Cheers
John