[7_14] Fix crash when pasting the source code of image

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## What
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```

## How to test your changes?
Copy and paste it to Mogan, it will crash Mogan.

src/Graphics/Fonts/smart_font.cpp

@@ -268,10 +268,15 @@ init_unicode_substitution () {
 
 int
 get_utf8_code (string c) {
+  int c_N= N (c);
+  if (c_N <= 2 || c_N > 6) {
+    // the largest unicode is U+10FFFF
+    return -1;
+  }
   string uc  = strict_cork_to_utf8 (c);
   int    pos = 0;
   int    code= decode_from_utf8 (uc, pos);
-  if (pos == N (uc)) return code;
+  if (pos == c_N) return code;
   else return -1;
 }
 

src/Plugins/Freetype/unicode_font.cpp

@@ -816,7 +816,11 @@ unicode_font_rep::index_glyph (string s, font_metric& rm, font_glyphs& rg) {
 
 static bool
 is_math_italic (string c) {
-  if (N (c) <= 2) return false;
+  int c_N= N (c);
+  if (c_N <= 2 || c_N > 6) {
+    // the largest unicode is U+10FFFF
+    return false;
+  }
   int i   = 0;
   int code= decode_from_utf8 (strict_cork_to_utf8 (c), i);
   if (code < 0x2100 || code > 0x1d7ff) return false;