Fix Windows 32-bit GN build.

R=tsepez@chromium.org,weili@chromium.org

Review URL: https://codereview.chromium.org/1879083002
diff --git a/core/fpdfdoc/cpvt_generateap.cpp b/core/fpdfdoc/cpvt_generateap.cpp
index 7b4ad2f..3c2b47c 100644
--- a/core/fpdfdoc/cpvt_generateap.cpp
+++ b/core/fpdfdoc/cpvt_generateap.cpp
@@ -370,7 +370,8 @@
             FX_BOOL bSelected = FALSE;
             if (pSels) {
               for (size_t s = 0, ssz = pSels->GetCount(); s < ssz; s++) {
-                if (i == pSels->GetIntegerAt(s)) {
+                int value = pSels->GetIntegerAt(s);
+                if (value >= 0 && i == static_cast<size_t>(value)) {
                   bSelected = TRUE;
                   break;
                 }