Prevent errors in sanitizeTTProgram, during parsing of CALL functions, when encountering invalid functions stack deltas (bug 1473809)

*I was feeling bored; so this is a very quick, and somewhat naive, attempt at fixing the bug.*

The breaking error, i.e. `Error during font loading: invalid array length`, was thrown when attempting to re-size the `stack` to a *negative* length when parsing the CALL functions.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1473809.

test/pdfs/.gitignore

@@ -151,6 +151,7 @@
 !bug1108301.pdf
 !bug1157493.pdf
 !bug1250079.pdf
+!bug1473809.pdf
 !pdfjsbad1586.pdf
 !freeculture.pdf
 !issue6006.pdf

test/test_manifest.json

@@ -143,6 +143,13 @@
        "lastPage": 4,
        "type": "eq"
     },
+    {  "id": "bug1473809",
+       "file": "pdfs/bug1473809.pdf",
+       "md5": "4b1ca51cf8cad58a1ce0618667341c76",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue2531",
       "file": "pdfs/issue2531.pdf",
       "md5": "c58e6642d8a6e2ddd5e07a543ef8f30d",