Small timezone bug fixed

Hi,

I was able to crash postgres 6.5.3 when I did an 'alter user' command. 
After I started a debugger I found the problem in the timezone handling of 
datetime (my Linux box lost its timezone information, that's how the 
problem occurred).

Only 7 bytes are reserved for the timezone, without checking for boundaries.

Attached is a patch that fixes this problem and emits a NOTICE if a 
timezone is encountered that is longer than MAXTZLEN bytes, like this:

template1=# alter user postgres with password postgres;
NOTICE:  Invalid timezone 'Local time zone must be set--see zic manual page'
NOTICE:  Invalid timezone 'Local time zone must be set--see zic manual page'
ALTER USER

I don't know whether the timezone should be reset to some predefined 
constant (like "GMT") if an error like this occurs. This patch at least 
directs the user in a general direction that something is wrong with his setup.

Cheers,

Jeroen