Fix some sign comparison warnings.

fpdfsdk/src/formfiller/FFL_ListBox.cpp

Index: fpdfsdk/src/formfiller/FFL_ListBox.cpp
diff --git a/fpdfsdk/src/formfiller/FFL_ListBox.cpp b/fpdfsdk/src/formfiller/FFL_ListBox.cpp
index 0735f48cf3afd795c0db54f6b53ef52dd4bb2b53..0f478fe7e0e3a4bb2dba20b539bfde5ad75dd991 100644
--- a/fpdfsdk/src/formfiller/FFL_ListBox.cpp
+++ b/fpdfsdk/src/formfiller/FFL_ListBox.cpp
@@ -101,13 +101,13 @@ FX_BOOL CFFL_ListBox::IsDataChanged(CPDFSDK_PageView* pPageView) {
     return FALSE;
 
   if (m_pWidget->GetFieldFlags() & FIELDFLAG_MULTISELECT) {
-    int nSelCount = 0;
+    size_t nSelCount = 0;
     for (int32_t i = 0, sz = pListBox->GetCount(); i < sz; ++i) {
       if (pListBox->IsItemSelected(i)) {
         if (m_OriginSelections.count(i) == 0)
           return TRUE;
 
-        nSelCount++;
+        ++nSelCount;
       }
     }