Update systemtap.base/utf_user_trunc.exp test case for gcc 7.

Update systemtap.base/utf_user_trunc.exp test case for gcc 7.

* testsuite/systemtap.base/utf_user_trunc.c: Lengthen MAXSTRINGLEN value
  to 41 characters to ensure that systmetap's error buffer is large enough
  to handle the "read fault" error messages (for example). Otherwise the
  test will fail when gcc 7 rightly complains that the error buffer isn't
  big enough.
* testsuite/systemtap.base/utf_user_trunc.stp: Ditto.