fix(grid-list): better handling of negative columns (#12939)

Currently if, by any chance, assigns a negative number to the `cols` of a grid list, they'll get a cryptic error like `Invalid array length`, because down-the-line we use that number to create an array. These changes ensure that the value is always a positive number.

Author: crisbeto

src/lib/grid-list/grid-list.spec.ts

@@ -1,5 +1,5 @@
 import {TestBed, ComponentFixture} from '@angular/core/testing';
-import {Component, DebugElement, Type} from '@angular/core';
+import {Component, DebugElement, Type, ViewChild} from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {MatGridList, MatGridListModule} from './index';
 import {MatGridTile, MatGridTileText} from './grid-tile';
@@ -34,6 +34,20 @@ describe('MatGridList', () => {
     expect(() => fixture.detectChanges()).toThrowError(/tile with colspan 5 is wider than grid/);
   });
 
+  it('should set the columns to zero if a negative number is passed in', () => {
+    const fixture = createComponent(GridListWithDynamicCols);
+    fixture.detectChanges();
+
+    expect(fixture.componentInstance.gridList.cols).toBe(2);
+
+    expect(() => {
+      fixture.componentInstance.cols = -2;
+      fixture.detectChanges();
+    }).not.toThrow();
+
+    expect(fixture.componentInstance.gridList.cols).toBe(1);
+  });
+
   it('should default to 1:1 row height if undefined ', () => {
     const fixture = createComponent(GridListWithUnspecifiedRowHeight);
     fixture.detectChanges();
@@ -334,6 +348,12 @@ class GridListWithInvalidRowHeightRatio { }
     '<mat-grid-list cols="4"><mat-grid-tile colspan="5"></mat-grid-tile></mat-grid-list>'})
 class GridListWithTooWideColspan { }
 
+@Component({template: '<mat-grid-list [cols]="cols"></mat-grid-list>'})
+class GridListWithDynamicCols {
+  @ViewChild(MatGridList) gridList: MatGridList;
+  cols = 2;
+}
+
 @Component({template: `
     <div style="width:200px">
       <mat-grid-list cols="1">

src/lib/grid-list/grid-list.ts

@@ -70,7 +70,9 @@ export class MatGridList implements OnInit, AfterContentChecked {
   /** Amount of columns in the grid list. */
   @Input()
   get cols(): number { return this._cols; }
-  set cols(value: number) { this._cols = Math.round(coerceNumberProperty(value)); }
+  set cols(value: number) {
+    this._cols = Math.max(1, Math.round(coerceNumberProperty(value)));
+  }
 
   /** Size of the grid list's gutter in pixels. */
   @Input()