Fix a static-analyzer warning.

frameworks/base/tools/aapt2/util/Files.cpp:81:21: warning: Null pointer
passed as an argument to a 'nonnull' parameter
     while ((entry = readdir(dir))) {

Bug: None.
Test: Warning no longer appears.
Change-Id: If242598405e995928378ce317a2e909654e1e985

tools/aapt2/util/Files.cpp

@@ -72,8 +72,8 @@ std::vector<std::string> listFiles(const StringPiece& root, std::string* outErro
             std::stringstream errorStr;
             errorStr << "unable to open file: " << strerror(errno);
             *outError = errorStr.str();
-            return {};
         }
+        return {};
     }
 
     std::vector<std::string> files;