Two functions have different ideas of what a string should look like;
one of them reads memory it assumes the other one may have written.
And "if you assume ..."
  
We now use a more defensive variety of the assuming function, this fixes
a warning thrown by the valgrind tool.


sql/item_cmpfunc.cc:
  c_ptr() makes incorrect assumptions about the string we get from
  out of args[0]->val_str(&tmp); c_str_safe() is more defensive.