Fixes an error with outros and elseifs

This is a fix for when an elseif doesn't have a final else, and the following error was thrown:

```
TypeError: Cannot read property 'o' of undefined
```

See https://svelte.technology/repl?version=2.12.0&gist=c33d308077447f8ba06b79d8ef5ab1e4

src/compile/nodes/IfBlock.ts

@@ -139,9 +139,10 @@ export default class IfBlock extends Node {
 				this.buildCompoundWithOutros(block, parentNode, parentNodes, branches, dynamic, vars);
 
 				if (this.compiler.options.nestedTransitions) {
-					block.builders.outro.addLine(
+					block.builders.outro.addBlock(deindent`
-						`${name}.o(#outrocallback);`
+						if (${name}) ${name}.o(#outrocallback);
-					);
+						else #outrocallback();
+					`);
 				}
 			} else {
 				this.buildCompound(block, parentNode, parentNodes, branches, dynamic, vars);

test/runtime/samples/if-block-outro-nested-else/Component.html

@@ -0,0 +1 @@
+<div></div>

test/runtime/samples/if-block-outro-nested-else/_config.js

@@ -0,0 +1,8 @@
+export default {
+	nestedTransitions: true,
+
+	test ( assert, component, target ) {
+		// Would cause "TypeError: Cannot read property 'o' of undefined"
+		component.set({ foo: false });
+	}
+};

test/runtime/samples/if-block-outro-nested-else/main.html

@@ -0,0 +1,16 @@
+{#if foo}
+	{#if false}
+		<Component/>
+	{:elseif false}
+		<Component/>
+	{/if}
+{/if}
+
+<script>
+	export default {
+		components: {
+			Component: './Component.html',
+		},
+		data: () => ({ foo: true }),
+	}
+</script>