Fix bug #18331 with "C-h k C-g" not showing documentation on Windows.

 src/data.c (set_internal): Use assq_no_quit, not Fassq, to find an
 existing binding of a variable, to avoid silently aborting
 commands that use specbind.

src/ChangeLog

+2014-09-04  Eli Zaretskii  <eliz@gnu.org>
+
+	* data.c (set_internal): Use assq_no_quit, not Fassq, to find an
+	existing binding of a variable, to avoid silently aborting
+	commands that use specbind.  (Bug#18331)
+
 2014-09-02  Eli Zaretskii  <eliz@gnu.org>
 
 	* dispnew.c (buffer_posn_from_coords): Fix an off-by-one error in

src/data.c

@@ -1241,10 +1241,10 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
 
 	    /* Find the new binding.  */
 	    XSETSYMBOL (symbol, sym); /* May have changed via aliasing.  */
-	    tem1 = Fassq (symbol,
-			  (blv->frame_local
-			   ? XFRAME (where)->param_alist
-			   : BVAR (XBUFFER (where), local_var_alist)));
+	    tem1 = assq_no_quit (symbol,
+				 (blv->frame_local
+				  ? XFRAME (where)->param_alist
+				  : BVAR (XBUFFER (where), local_var_alist)));
 	    set_blv_where (blv, where);
 	    blv->found = 1;