Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/gc/space/malloc_space.cc b/runtime/gc/space/malloc_space.cc
index 9195b06..b014217 100644
--- a/runtime/gc/space/malloc_space.cc
+++ b/runtime/gc/space/malloc_space.cc
@@ -75,13 +75,13 @@
     LOG(ERROR) << "Failed to create alloc space (" << name << ") where the initial size ("
         << PrettySize(*initial_size) << ") is larger than its capacity ("
         << PrettySize(*growth_limit) << ")";
-    return NULL;
+    return nullptr;
   }
   if (*growth_limit > *capacity) {
     LOG(ERROR) << "Failed to create alloc space (" << name << ") where the growth limit capacity ("
         << PrettySize(*growth_limit) << ") is larger than the capacity ("
         << PrettySize(*capacity) << ")";
-    return NULL;
+    return nullptr;
   }
 
   // Page align growth limit and capacity which will be used to manage mmapped storage