Enable/silence -Wsign-conversion.

llvm-svn: 152323
diff --git a/libcxxabi/src/fallback_malloc.ipp b/libcxxabi/src/fallback_malloc.ipp
index 979f0bb..ec75e88 100644
--- a/libcxxabi/src/fallback_malloc.ipp
+++ b/libcxxabi/src/fallback_malloc.ipp
@@ -54,7 +54,7 @@
     { return (heap_node *) ( heap + ( offset * sizeof (heap_node))); }
 
 heap_offset offset_from_node ( const heap_node *ptr )
-    { return (((char *) ptr ) - heap)  / sizeof (heap_node); }
+    { return static_cast<size_t>(((char *) ptr ) - heap)  / sizeof (heap_node); }
  
 void init_heap () {
     freelist = (heap_node *) heap;