Fix exception when resizing both dimensions

This happened because buffers now resize themselves but they were
relying on Terminal to give them a blank line. The blank line was
coming back with the old columns value, causing an NPE.

Fixes #860

src/Buffer.test.ts

@@ -143,5 +143,16 @@ describe('Buffer', () => {
         });
       });
     });
+
+    describe('row and column increased', () => {
+      it('should resize properly', () => {
+        buffer.fillViewportRows();
+        buffer.resize(INIT_COLS + 5, INIT_ROWS + 5);
+        assert.equal(buffer.lines.length, INIT_ROWS + 5);
+        for (let i = 0; i < INIT_ROWS + 5; i++) {
+          assert.equal(buffer.lines.get(i).length, INIT_COLS + 5);
+        }
+      });
+    });
   });
 });