[std] Redefine \@ in PDF output

New uses of `\@` were added by e1f9b90 to improve spacing after a full stop. But this command should not be used in PDF output.

Author: morinmorin

source/std.tex

@@ -83,6 +83,7 @@
 \pdfstringdefDisableCommands{\def\raisebox#1{}}
 \pdfstringdefDisableCommands{\def\hspace#1{}}
 \pdfstringdefDisableCommands{\def\frenchspacing{}}
+\pdfstringdefDisableCommands{\def\@{}}
 
 %%--------------------------------------------------
 %% add special hyphenation rules