Prevent trying to map characters to the specials unicode block in adjustMapping (issue 4650)

src/core/fonts.js

@@ -2459,6 +2459,8 @@ var Font = (function FontClosure() {
            fontCharCode === 0x7F || // Control char
            fontCharCode === 0xAD || // Soft hyphen
            (fontCharCode >= 0x80 && fontCharCode <= 0x9F) || // Control chars
+           // Prevent drawing characters in the specials unicode block.
+           (fontCharCode >= 0xFFF0 && fontCharCode <= 0xFFFF) ||
            (isSymbolic && isIdentityUnicode)) &&
           nextAvailableFontCharCode <= PRIVATE_USE_OFFSET_END) { // Room left.
         // Loop to try and find a free spot in the private use area.

test/pdfs/.gitignore

@@ -64,6 +64,7 @@
 !noembed-sjis.pdf
 !vertical.pdf
 !bug878026.pdf
+!issue4650.pdf
 !issue3025.pdf
 !issue2099-1.pdf
 !issue3371.pdf

test/test_manifest.json

@@ -430,6 +430,15 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue4650",
+       "file": "pdfs/issue4650.pdf",
+       "md5": "ad736804f57f9f96f5ac108e514e1686",
+       "rounds": 1,
+       "link": false,
+       "firstPage": 1,
+       "lastPage": 1,
+       "type": "eq"
+    },
     {  "id": "txt2pdf",
        "file": "pdfs/txt2pdf.pdf",
        "md5": "02cefa0f5e8d96313bb05163b2f88c8c",