sql: add '\0' to the BLOB when it is cast to INTEGER

Prior to this patch, due to the absence of the '\0' character at
the end of the BLOB, it was possible to get an error or incorrect
result when using CAST() from BLOB to INTEGER or UNSIGNED. This
has now been fixed, but the maximum length of a BLOB that could be
cast to INTEGER or UNSIGNED was limited to 12287 bytes.

Examples of wrong CAST() from BLOB to INTEGER:

CREATE TABLE t (i INT PRIMARY KEY, a VARBINARY, b INT, c INT);
INSERT INTO t VALUES (1, X'33', 0x33, 0x00), (2, X'34', 0x41, 0);

Example of wrong result:

SELECT CAST(a AS INTEGER) FROM t WHERE i = 1;

Result: 33

Example of error during CAST():

SELECT CAST(a AS INTEGER) FROM t WHERE i = 2;

Result: 'Type mismatch: can not convert varbinary to integer'

Closes #4766