On exactly-sized Strings, the String::c_ptr() function peeked beyond the
end of the buffer, possibly into unititialized space to see whether the 
buffer was NUL-terminated.

In a place that did peek improperly, we now use a c_ptr_safe() function, 
which doesn't peek where it shouldn't.


client/sql_string.h:
  Back-port String::c_ptr_safe().
sql/item_func.h:
  Describe side-effect behavior.
sql/item_strfunc.cc:
  Use the "_safe" version of c_ptr to avoid looking for a terminating 
  NUL character outside the initialized memory area.  Valgrind hates it 
  when one does that, and it theoretically could lead to a SEGV.
sql/sql_string.h:
  Back-port String::c_ptr_safe().