cid#1606852 silence Overflowed constant

Change-Id: I94d75d6c2b69937e5d0f29b498518999b1f49120
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/177167
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Tested-by: Jenkins

registry/source/reflwrit.cxx

@@ -62,6 +62,8 @@ sal_uInt32 readString(const sal_uInt8* buffer, sal_Unicode* v, sal_uInt32 maxSiz
         len = maxSize / 2;
     }
 
+    assert(len > 0 && "maxSize has to be >= 2");
+
     for (i = 0; i < len; i++)
     {
         sal_uInt16 aChar;