degrib myutil.c strTrimRight no longer working with r40749

[Kurt Schwehr]

third_party/gdal/autotest2/cpp/frmts/grib/degrib/myutil_test.cc:115
Expected equality of these values:
  "a"
  a
    Which is: "a "

​https://github.com/schwehr/gdal-autotest2/blob/master/cpp/frmts/grib/degrib/myutil_test.cc#L115

  char a[] = "a ";
  strTrimRight(a, 'z');
  EXPECT_STREQ("a", a);

[Even Rouault]

In 40804:

Fix strTrimRight() regression due to cast put at inappropriate place during degrib 2.14 merging (fixes #7162)