Avoid CHECK in fpdf_parser_decode.cpp (memcpy empty span)

Given a span of size N, memcpy(dest, &span[N], 0) ought to be a no-op,
but since we compute span[N] before checking for zero length, we hit
an assert. The correct idiom should be to create a sub-span, which
allows specifying N, but only when the size is 0.

Bug: 879910
Change-Id: Ic6f368109a5c2f1e13a5f638c6a233769e2ad41b
Reviewed-on: https://pdfium-review.googlesource.com/41930
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

core/fpdfapi/parser/fpdf_parser_decode.cpp

diff --git a/core/fpdfapi/parser/fpdf_parser_decode.cpp b/core/fpdfapi/parser/fpdf_parser_decode.cpp
index de93acf..410c9a9 100644
--- a/core/fpdfapi/parser/fpdf_parser_decode.cpp
+++ b/core/fpdfapi/parser/fpdf_parser_decode.cpp
@@ -239,7 +239,8 @@
         copy_len = buf_left;
         memset(*dest_buf + dest_count + copy_len, '\0', delta);
       }
-      memcpy(*dest_buf + dest_count, &src_span[i + 1], copy_len);
+      auto copy_span = src_span.subspan(i + 1, copy_len);
+      memcpy(*dest_buf + dest_count, copy_span.data(), copy_span.size());
       dest_count += src_span[i] + 1;
       i += src_span[i] + 2;
     } else {