Check PDF_CharNameFromPredefinedCharSet() never returns an empty string.

Change-Id: I947d3b785e094a15b40d5831fe16f6dad740b1f7
Reviewed-on: https://pdfium-review.googlesource.com/c/48153
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

core/fpdfapi/font/cpdf_font.cpp

diff --git a/core/fpdfapi/font/cpdf_font.cpp b/core/fpdfapi/font/cpdf_font.cpp
index 7867a79..369c1bd 100644
--- a/core/fpdfapi/font/cpdf_font.cpp
+++ b/core/fpdfapi/font/cpdf_font.cpp
@@ -359,7 +359,11 @@
   const char* name = nullptr;
   if (iBaseEncoding)
     name = PDF_CharNameFromPredefinedCharSet(iBaseEncoding, charcode);
-  return name && name[0] ? name : nullptr;
+  if (!name)
+    return nullptr;
+
+  ASSERT(name[0]);
+  return name;
 }
 
 uint32_t CPDF_Font::FallbackFontFromCharcode(uint32_t charcode) {