memory reference

There are two issues present here.
  1) There is a possibility that we test a byte beyond the
     allocated buffer

  2) We compare a byte that might never have been
     initalized to see if it's 0.

The first issue is not triggered by existing code, but an
ASSERT has been added to safe-guard against introducing
new code that triggers it.

The second issue is what triggers the Valgrind warnings
reported in the bug report. A buffer is allocated in
class String to hold the value. This buffer is populated
by the character data constituting the string, but is not
zero-terminated in most cases.  Testing if it is indeed
zero-terminated means that we check a byte that has never
been explicitly set, thus causing Valgrind to trigger.

Note that issue 2 is not a serious problem.  The variable
is read, and if it's not zero, we will set it to zero.
There are no further consequences.

Note that this patch does not fix the underlying problems
with issue 1, as it is deemed too risky to fix at this
point (as noted in the bug report).  As discussed in
the report, the c_ptr() method should probably be
replaced, but this requires a thorough analysis of the
~200 calls to the method.


sql/set_var.cc:
  These two cases have been reported to fail
  with Valgrind.