patch 8.2.3548: GTK GUI crashen when reading from stdin

Problem:    GTK GUI crashen when reading from stdin.
Solution:   Do not overwrite the NUL after the string. (closes #9028)

src/gui_gtk_x11.c

@@ -5744,11 +5744,15 @@ gui_gtk2_draw_string(int row, int col, char_u *s, int len, int flags)
 	    }
 	}
 
+	if (slen < len)
+	{
 	    // temporarily zero terminate substring, print, restore char, wrap
 	    backup_ch = *(cs + slen);
-	*(cs + slen) = 0;
+	    *(cs + slen) = NUL;
+	}
 	len_sum += gui_gtk2_draw_string_ext(row, col + len_sum,
 						 cs, slen, flags, needs_pango);
+	if (slen < len)
 	    *(cs + slen) = backup_ch;
 	cs += slen;
 	byte_sum += slen;

src/testdir/test_gui.vim

@@ -145,6 +145,28 @@ func Test_quoteplus()
   let @+ = quoteplus_saved
 endfunc
 
+func Test_gui_read_stdin()
+  CheckUnix
+
+  call writefile(['some', 'lines'], 'Xstdin')
+  let script =<< trim END
+      call writefile(getline(1, '$'), 'XstdinOK')
+      qa!
+  END
+  call writefile(script, 'Xscript')
+
+  " Cannot use --not-a-term here, the "reading from stdin" message would not be
+  " displayed.
+  let vimcmd = substitute(GetVimCommand(), '--not-a-term', '', '')
+
+  call system('cat Xstdin | ' .. vimcmd .. ' -f -g -S Xscript -')
+  call assert_equal(['some', 'lines'], readfile('XstdinOK'))
+
+  call delete('Xstdin')
+  call delete('XstdinOK')
+  call delete('Xscript')
+endfunc
+
 func Test_set_background()
   let background_saved = &background
 

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3548,
 /**/
     3547,
 /**/