Fix bug #54379 PDO_OCI: UTF-8 output gets truncated

Oracle docs defines three versions of utf8 encoding.

- AL32UTF8 and UTFE character can occupy up to 4 bytes.
- UTF8 character can occupy up to 6 bytes.
Details here: http://docs.oracle.com/cd/B19306_01/server.102/b14225/ch6unicode.htm#i1006998 
We can obtain maximum character size in bytes by calling OCINlsNumericInfoGet() and get current max character width. 
Details here: http://docs.oracle.com/cd/B10501_01/server.920/a96529/ch8.htm#14288