Fix underflow bug in rewrite_call

Fixes https://github.com/nrc/rustfmt/issues/148.
Now properly propagates the rewrite failure instead of panicking. Added regression test from servo code. This example will be properly rewritten when https://github.com/nrc/rustfmt/issues/17 is addressed.

src/expr.rs

@@ -132,7 +132,7 @@ fn rewrite_call(context: &RewriteContext,
     }
 
     // 2 is for parens.
-    let remaining_width = width - callee_str.len() - 2;
+    let remaining_width = try_opt!(width.checked_sub(callee_str.len() + 2));
     let offset = callee_str.len() + 1 + offset;
 
     let items = itemize_list(context.codemap,

tests/target/fn.rs

@@ -79,3 +79,17 @@ fn main() {
     let _ = function(move || 5);
     let _ = move || 42;
 }
+
+fn servo() {
+    let constellation_chan = Constellation::<layout::layout_task::LayoutTask,
+    script::script_task::ScriptTask>::start(
+        compositor_proxy,
+        resource_task,
+        image_cache_task,
+        font_cache_task,
+        time_profiler_chan,
+        mem_profiler_chan,
+        devtools_chan,
+        storage_task,
+        supports_clipboard);
+}