Avoid GCC 8.X strncpy() errors in JFR code

Gcc 8 has strict (and sometimes buggy) tests for strncpy() misuse.

A common pattern in the JDK is:

   const size_t not_impl_len = strlen(not_impl);
   *str = NEW_C_HEAP_ARRAY(char, not_impl_len+1, mtTracing);
   strncpy(*str, not_impl, not_impl_len);
   (*str)[not_impl_len] = '\0';

More efficient code that does not cause GCC warnings would use strncpy() to add the trailing null, resulting in shorter code:

   strncpy(*str, not_impl, not_impl_len);
   (*str)[not_impl_len] = '\0';

becomes:

   strncpy(*str, not_impl, not_impl_len+1);

(found by compiling with GCC 8.1)