Fallback on default color when linkifier can't find the character

This probably doesn't fix the underlying problem, but it will make it
obvious by using the white background color and more easily help identify
the problem case.

Fixes #1669

src/Linkifier.ts

@@ -220,8 +220,11 @@ export class Linkifier extends EventEmitter implements ILinkifier {
     // Get cell color
     const line = this._terminal.buffer.lines.get(this._terminal.buffer.ydisp + rowIndex);
     const char = line.get(index);
-    const attr: number = char[CHAR_DATA_ATTR_INDEX];
-    const fg = (attr >> 9) & 0x1ff;
+    let fg: number | undefined;
+    if (char) {
+      const attr: number = char[CHAR_DATA_ATTR_INDEX];
+      fg = (attr >> 9) & 0x1ff;
+    }
 
     // Ensure the link is valid before registering
     if (matcher.validationCallback) {