Check length in CFX_Win32FontInfo::AddInstalledFont

This CL is a tentative fix for the bug below. The last method in the
stack trace is FontEnumProc, which calls AddInstalledFont. This problem
was introduced by
https://pdfium-review.googlesource.com/c/pdfium/+/10651

Bug: chromium:757388
Change-Id: If089a43367f1106a79276f6b09b6d0546b863e35
Reviewed-on: https://pdfium-review.googlesource.com/11611
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>
diff --git a/core/fxge/win32/fx_win32_device.cpp b/core/fxge/win32/fx_win32_device.cpp
index 101c945..5f778f8 100644
--- a/core/fxge/win32/fx_win32_device.cpp
+++ b/core/fxge/win32/fx_win32_device.cpp
@@ -430,7 +430,7 @@
 void CFX_Win32FontInfo::AddInstalledFont(const LOGFONTA* plf,
                                          uint32_t FontType) {
   CFX_ByteString name(plf->lfFaceName);
-  if (name[0] == '@')
+  if (name.GetLength() > 0 && name[0] == '@')
     return;
 
   if (name == m_LastFamily) {