Bug #14057034 : WASTED CPU CYCLES IN MY_UTF8_UNI WHERE

                RESULTING MY_WC_T RESULT IS NOT USED
Issue         : handler functions my_ismbchar_utf8,
              my_well_formed_len_mb for charset utf8
              is calling unicode converion function
              to validate and to find the character
              length. Because of this, instructions
              which will convert the utf8 to unicode
              are executed for no use.
              A similar issue exist with charset utf8mb4
Solution      : reorganized the code such that character
              validation part of unicode conversion
              handler is extracted(duplicated) in to
              separate function. Hence
              my_ismbchar_utf8, my_well_formed_len_mb
              will call the new function which only
              validates and return the length of mb(utf8).
              A similar fix for charset utf8mb4.