Use explicit null check to ensure start/end row is valid

It looks like this could have caused the inconsistent behavior with
links not activating and then working again after scrolling

Fixes xtermjs#1233

src/Linkifier.ts

@@ -82,12 +82,12 @@ export class Linkifier extends EventEmitter implements ILinkifier {
     }
 
     // Increase range to linkify
-    if (!this._rowsToLinkify.start) {
+    if (this._rowsToLinkify.start === null) {
       this._rowsToLinkify.start = start;
       this._rowsToLinkify.end = end;
     } else {
-      this._rowsToLinkify.start = this._rowsToLinkify.start < start ? this._rowsToLinkify.start : start;
-      this._rowsToLinkify.end = this._rowsToLinkify.end > end ? this._rowsToLinkify.end : end;
+      this._rowsToLinkify.start = Math.min(this._rowsToLinkify.start, start);
+      this._rowsToLinkify.end = Math.max(this._rowsToLinkify.end, end);
     }
 
     // Clear out any existing links on this row range
@@ -97,7 +97,7 @@ export class Linkifier extends EventEmitter implements ILinkifier {
     if (this._rowsTimeoutId) {
       clearTimeout(this._rowsTimeoutId);
     }
-    this._rowsTimeoutId = <number><any>setTimeout(() => this._linkifyRows(), Linkifier.TIME_BEFORE_LINKIFY);
+    this._rowsTimeoutId = window.setTimeout(() => this._linkifyRows(), Linkifier.TIME_BEFORE_LINKIFY);
   }
 
   /**