Fix staticcheck error about possible nil dereference. (#492)

internal/driver/interactive_test.go

 		// Get report output format
 		c := pprofCommands[cmd[0]]
 		if c == nil {
-			t.Errorf("unexpected nil command")
+			t.Fatalf("unexpected nil command")
 		}
 		vars = applyCommandOverrides(cmd[0], c.format, vars)