Add a std::string overload to Next.

All but one existing caller actually wants a std::string.

Bug: http://b/129068177
Test: treehugger
Change-Id: I428c4453edaae74451db56e9542e4e462f08d43a
diff --git a/libziparchive/unzip.cpp b/libziparchive/unzip.cpp
index 3a3a694..426325e 100644
--- a/libziparchive/unzip.cpp
+++ b/libziparchive/unzip.cpp
@@ -255,9 +255,8 @@
   }
 
   ZipEntry entry;
-  ZipString string;
-  while ((err = Next(cookie, &entry, &string)) >= 0) {
-    std::string name(string.name, string.name + string.name_length);
+  std::string name;
+  while ((err = Next(cookie, &entry, &name)) >= 0) {
     if (ShouldInclude(name)) ProcessOne(zah, entry, name);
   }