Compute the loca table endOffset, of the "first" glyph, correctly (issue 14618)

When there are *multiple* empty glyphs at the start of the data, ensure that the "first" glyph gets a correct `endOffset` to avoid skipping it during parsing in the `sanitizeGlyph` function.

test/pdfs/.gitignore

@@ -321,6 +321,7 @@
 !jbig2_symbol_offset.pdf
 !gradientfill.pdf
 !bug903856.pdf
+!issue14618.pdf
 !bug850854.pdf
 !issue12810.pdf
 !bug866395.pdf

test/test_manifest.json

@@ -351,6 +351,12 @@
        "rounds": 1,
        "type": "load"
     },
+    {  "id": "issue14618",
+       "file": "pdfs/issue14618.pdf",
+       "md5": "8a4a5aac59519228019aaffdfbd56f17",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "intelisa-eq",
        "file": "pdfs/intelisa.pdf",
        "md5": "24643ebe348a568cfe6a532055c71493",