This issue has been found by Coverity - static analysis tool.
mysql-5.5.31/strings/ctype-ucs2.c:1707:sign_extension – Suspicious implicit sign extension: "s[0]" with type "unsigned char" (8 bits, unsigned) is promoted in "(s[0] << 24) + (s[1] << 16) + (s[2] << 8) + s[3]" to type "int" (32 bits, signed), then sign-extended to type "unsigned long" (64 bits, unsigned). If "(s[0] << 24) + (s[1] << 16) + (s[2] << 8) + s[3]" is greater than 0x7FFFFFFF, the upper bits of the result will all be 1.