Replace the UNSAFE_TODO() in FPDFAnnot_GetFormFieldExportValue()

It should have been UNSAFE_BUFFERS(), as there is already a "SAFETY"
comment.

Change-Id: I6a793e74de26f946ac8d0f3d7c6e1a7489bf3688
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/134850
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

fpdfsdk/fpdf_annot.cpp

diff --git a/fpdfsdk/fpdf_annot.cpp b/fpdfsdk/fpdf_annot.cpp
index 08e9250..a676142 100644
--- a/fpdfsdk/fpdf_annot.cpp
+++ b/fpdfsdk/fpdf_annot.cpp
@@ -1707,7 +1707,7 @@
   // SAFETY: required from caller.
   return Utf16EncodeMaybeCopyAndReturnLength(
       pWidget->GetExportValue(),
-      UNSAFE_TODO(SpanFromFPDFApiArgs(buffer, buflen)));
+      UNSAFE_BUFFERS(SpanFromFPDFApiArgs(buffer, buflen)));
 }
 
 FPDF_EXPORT FPDF_BOOL FPDF_CALLCONV FPDFAnnot_SetURI(FPDF_ANNOTATION annot,