cppcheck: unusedFunction

Change-Id: I81f2a4be65bbc9359e51accccb746f1e8ea48c4f
WaE: warning C4146: unary minus operator applied to unsigned type,...

...result still unsigned at sw/source/filter/ww8/docxsdrexport.cxx(441)

Just use SAL_{MAX,MIN}_INT32 (change -2147483648 to (-2147483647-1)) which fixes this warning for MSVC.

Change-Id: Ia039906602d3b00193a364bddb81626afa928f2d