Fix RTLD_NEXT lookup for the local_group

Linker used to skip RTLD_NEXT within local_group when
  caller->next is nullptr (which represents load order
  but not the order within local_group dependency tree)

Bug: http://b/21380474
Change-Id: I178fc4657b19bceb871635b177c1df67855b1708
(cherry picked from commit b96ac41b32)

linker/linker.cpp

@@ -980,7 +980,7 @@ const ElfW(Sym)* dlsym_linear_lookup(const char* name,
   soinfo* start = solist;
 
   if (handle == RTLD_NEXT) {
-    if (caller == nullptr || caller->next == nullptr) {
+    if (caller == nullptr) {
       return nullptr;
     } else {
       start = caller->next;