vm.c: remove unnecessary branch

th->altstack is never NULL, and even if it were, POSIX
stipulates free(3) on NULL to be a no-op.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62031 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm.c

@@ -2439,9 +2439,7 @@ thread_free(void *ptr)
     }
     else {
 #ifdef USE_SIGALTSTACK
-	if (th->altstack) {
+	free(th->altstack);
-	    free(th->altstack);
-	}
 #endif
 	ruby_xfree(ptr);
     }