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
1 file changed