Remove unnecessary AssertMacro() to suppress gcc 4.6 compiler warning.

Remove unnecessary AssertMacro() to suppress gcc 4.6 compiler warning.

There's no particular value in doing AssertMacro((tup) != NULL) in front
of code that's certain to crash anyway if tup is NULL.  And if "tup" is
actually the address of a local variable, gcc 4.6 whinges about it.  That's
arguably pretty broken on gcc's part, but we might as well remove the
useless test to silence the warnings.  This gets rid of all the -Waddress
warnings in the backend; there are some in libpq and psql that are a bit
harder to avoid.