Upgrade to mksh 50f.

Bug: http://b/20101805
Change-Id: Ic311851b31217517f6c201b5ec68a0668577eddf
diff --git a/Android.mk b/Android.mk
index e208225..a78409f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -79,6 +79,6 @@
     -DHAVE_SETGROUPS=1 -DHAVE_STRERROR=1 -DHAVE_STRSIGNAL=0 \
     -DHAVE_STRLCPY=1 -DHAVE_FLOCK_DECL=1 -DHAVE_REVOKE_DECL=1 \
     -DHAVE_SYS_ERRLIST_DECL=0 -DHAVE_SYS_SIGLIST_DECL=1 \
-    -DHAVE_PERSISTENT_HISTORY=0 -DMKSH_BUILD_R=505
+    -DHAVE_PERSISTENT_HISTORY=0 -DMKSH_BUILD_R=506
 
 include $(BUILD_EXECUTABLE)