Fix ubsan failure in android::base::Split.

ubsan doesn't like us incrementing std::string::npos, even if we won't use
the result.

Bug: http://b/28729303
Test: ran tests
Change-Id: I8227eca57dc6f3e49626c7025514caa04ef18f0a
diff --git a/strings_test.cpp b/strings_test.cpp
index 7a65a00..7ed5b2b 100644
--- a/strings_test.cpp
+++ b/strings_test.cpp
@@ -251,3 +251,7 @@
   ASSERT_FALSE(android::base::EqualsIgnoreCase("foo", "bar"));
   ASSERT_FALSE(android::base::EqualsIgnoreCase("foo", "fool"));
 }
+
+TEST(strings, ubsan_28729303) {
+  android::base::Split("/dev/null", ":");
+}