ofz#2877: crash in SVTB16Short

sal_uInt16 wraparound

Change-Id: Ifd791bdd5f1b96576fdd4ca6665bb972fb8b1e4c
Reviewed-on: https://gerrit.libreoffice.org/40848Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/filter/ww8/ww8scan.cxx

@@ -3493,7 +3493,7 @@ bool WW8PLCFx_SEPX::Find4Sprms(sal_uInt16 nId1,sal_uInt16 nId2,sal_uInt16 nId3,s
     bool bFound = false;
 
     sal_uInt8* pSp = pSprms.get();
-    sal_uInt16 i=0;
+    size_t i = 0;
     while (i + maSprmParser.MinSprmLen() <= nSprmSiz)
     {
         // Sprm found?