Wine was not doing bounds checks for EM_GETTEXTRANGE, which was causing
a crash in Bug 17822.  The added tests would cause a crash without the
added bounds checks in the richedit code.

The bounds checks I put in HandleMessage, since ME_GetTextRange is also
called for ME_GETSELTEXT which should not have bounds checks, since it
uses the selection range.

When the ME_GETTEXTRANGE message returns 0, no text is copied, not even
the NULL terminating charter.  This differs from EM_GETSELTEXT which
will copy the NULL terminating character when no text is selected.  This
behaviour is consistent with native richedit controls.