the value obtained by String::c_ptr() method not always
    has the ending zero. Particularly in this bug the
    dlsym() expects zero-ending string.
    The String::c_ptr_safe() is more correct here.

per-file comments:
  sql/item_func.cc
Bug#46570      test udf fails with valgrind
    c_ptr_safe() used for UDF parameters as the library expects
    zero-ending strings.