Fix a sign / unsigned comparison in CFGAS_StringFormatter.

GCC warns about this.

Change-Id: If5ba69ff710cce474b23c262283d66eaace15588
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/52290
Auto-Submit: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/xfa/fgas/crt/cfgas_stringformatter.cpp b/xfa/fgas/crt/cfgas_stringformatter.cpp
index 6264510..8594e07 100644
--- a/xfa/fgas/crt/cfgas_stringformatter.cpp
+++ b/xfa/fgas/crt/cfgas_stringformatter.cpp
@@ -2030,7 +2030,7 @@
   }
 
   if (cc < spSrcNum.size()) {
-    int nPos = dot_index.value() % 3;
+    size_t nPos = dot_index.value() % 3;
     wsOutput->clear();
     for (size_t i = 0; i < dot_index.value(); i++) {
       if (i % 3 == nPos && i != 0)