sw: -Wimplicit-fallthrough from MacOS clang (or perhaps NDEBUG clang?)

Change-Id: I0219a0f6438c58b3f3307efe0b62d2a40ef281e4

sw/source/core/fields/reffld.cxx

@@ -559,6 +559,7 @@ void SwGetRefField::UpdateField( const SwTextField* pFieldTextAttr )
 
                 default:
                     assert(false); // fall through to appease MSVC C4701
+                    SAL_FALLTHROUGH;
                 // "Reference" (whole Text)
                 case REF_CONTENT:
                     nStart = 0;
@@ -600,6 +601,7 @@ void SwGetRefField::UpdateField( const SwTextField* pFieldTextAttr )
 
             default:
                 assert(false); // fall through to appease MSVC C4701
+                SAL_FALLTHROUGH;
             case REF_SETREFATTR:
                 nStart = nNumStart;
                 nEnd = nNumEnd;