fix(select): open method triggered twice on click (#7619)

Fixes the `open` method being triggered both by a click handler on the select trigger and the `onContainerClick` from the `MatFormField` integration.

Author: crisbeto

src/lib/select/select.ts

@@ -508,7 +508,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
 
   /** Opens the overlay panel. */
   open(): void {
-    if (this.disabled || !this.options || !this.options.length) {
+    if (this.disabled || !this.options || !this.options.length || this._panelOpen) {
       return;
     }