Fix off-by-one introduced at ab20404

Building a little-endian n-byte value via a single byte
store and a memset(0) is dubious, but don't clobber the
value we just stored.

Bug: 723625
Change-Id: I015ea3e01c63a534f2ca0a1c085ed67777330d82
Reviewed-on: https://pdfium-review.googlesource.com/5732
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fpdfapi/font/fpdf_font_cid.cpp b/core/fpdfapi/font/fpdf_font_cid.cpp
index e451546..dd37551 100644
--- a/core/fpdfapi/font/fpdf_font_cid.cpp
+++ b/core/fpdfapi/font/fpdf_font_cid.cpp
@@ -687,7 +687,7 @@
           iSize = 1;
         str[iSize - 1] = (uint8_t)charcode;
         if (iSize > 1)
-          memset(str + 1, 0, iSize - 1);
+          memset(str, 0, iSize - 1);
         return iSize;
       }
       if (charcode < 0x10000) {