Replace a magic number with its syntactic meaning

Change-Id: I535a7317f0797ef63294df05463a88e9e8d7aa10
Reviewed-on: https://gerrit.libreoffice.org/43213Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

sw/source/core/unocore/unofield.cxx

@@ -2652,7 +2652,7 @@ SwXTextFieldMasters::~SwXTextFieldMasters()
 static SwFieldIds lcl_GetIdByName( OUString& rName, OUString& rTypeName )
 {
     if (rName.startsWithIgnoreAsciiCase(COM_TEXT_FLDMASTER_CC))
-        rName = rName.copy(30);
+        rName = rName.copy(RTL_CONSTASCII_LENGTH(COM_TEXT_FLDMASTER_CC));
 
     SwFieldIds nResId = SwFieldIds::Unknown;
     sal_Int32 nFound = 0;