Avoid pointer arithmetic in string_view_template.h.

There already is a span, so the code is simpler in terms of subspan(),
and avoids a clang warning from experimental flag -Wunsafe_buffer_usage.

Change-Id: Id7f3bb669a873c502f043f15799be4e8557517fd
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/109570
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxcrt/string_view_template.h b/core/fxcrt/string_view_template.h
index 67cd3ff..8a2c6b2 100644
--- a/core/fxcrt/string_view_template.h
+++ b/core/fxcrt/string_view_template.h
@@ -214,7 +214,7 @@
     if (!IsValidIndex(first + count - 1))
       return StringViewTemplate();
 
-    return StringViewTemplate(m_Span.data() + first, count);
+    return StringViewTemplate(m_Span.subspan(first, count));
   }
 
   StringViewTemplate First(size_t count) const {