Enhance getcwd(3) to handle NULL like glibc.

Bug: http://code.google.com/p/android/issues/detail?id=36085
Change-Id: I960a1b585887eb66176c61d29c5c61c239a4003f
diff --git a/tests/Android.mk b/tests/Android.mk
index 2721138..e9fdde1 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -20,6 +20,7 @@
 test_tags = eng tests
 
 test_src_files = \
+    getcwd_test.cpp \
     regex_test.cpp \
 
 # Build for the device (with bionic). Run with: