odbc: Fix stack corruption in get_diagnos in odbcserver

SQLGetDiagRec can fill output buffer and return SQL_SUCCESS_WITH_INFO.
In that case we can not use strcat on diagnos.error_msg as it will write
outside allocated space.
Correctly set acc_errmsg_size in such case.

See also ERL-808 at bugs.erlang.org.

The regression was introduced by making byte type signed.

Bitwise OR (|) applied to signed char (byte) and any longer type reqires the former to expand to the same size.
When the char is negative it is expanded with sign bit filling the extra bytes causing the unexpected result.

These are signaled by -Wall

Three different pointer types are used for strings.

SQLCHAR pointer - used against ODBC driver
char pointer - used against erl_interface
byte pointer - used internally

SQLCHAR is 'unsigned char' while erl_interface is 'signed char'

byte pointer should probably not be used at all.

odbcserver.c:2772:12: warning: implicit declaration of function 'tolower' is invalid in C99
      [-Wimplicit-function-declaration]
                str[i] = tolower(str[i]);
                         ^