Better types for nFc and nLen

...given that they get assigned with values of those types.  This change has no
actual effect on the results, but silences Clang
-fsanitize=implicit-signed-integer-truncation warnings like "implicit conversion
from type 'long' of value -671088640 (64-bit, signed) to type 'sal_uInt32' (aka
'unsigned int') changed the value to 3623878656 (32-bit, unsigned)".

Change-Id: I763a9d85b9e40797312688551206c308fc3d36e3
Reviewed-on: https://gerrit.libreoffice.org/68705
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sw/source/filter/ww8/ww8scan.cxx

@@ -3953,7 +3953,8 @@ void WW8PLCFx_SubDoc::advance()
 WW8PLCFx_FLD::WW8PLCFx_FLD( SvStream* pSt, const WW8Fib& rMyFib, short nType)
     : WW8PLCFx(rMyFib, true), rFib(rMyFib)
 {
-    long nFc, nLen;
+    WW8_FC nFc;
+    sal_Int32 nLen;
 
     switch( nType )
     {