and for bug #31070: crash during conversion of charsets

Problem: passing a 0 byte length string to some my_mb_wc_XXX() 
functions leads to server crash due to improper argument check.

Fix: properly check arguments passed to my_mb_wc_XXX() functions.


mysql-test/include/ctype_common.inc:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - test case.
mysql-test/r/ctype_big5.result:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - test result.
mysql-test/r/ctype_euckr.result:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - test result.
mysql-test/r/ctype_gb2312.result:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - test result.
mysql-test/r/ctype_gbk.result:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - test result.
mysql-test/r/ctype_uca.result:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - test result.
strings/ctype-big5.c:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - check the string length before testing its first byte.
strings/ctype-cp932.c:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - check the string length before testing its first byte.
strings/ctype-euc_kr.c:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - check the string length before testing its first byte.
strings/ctype-gb2312.c:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - check the string length before testing its first byte.
strings/ctype-sjis.c:
  Fix for bug #31069: crash in 'sounds like'
  and bug #31070: crash during conversion of charsets
    - check the string length before testing its first byte.