liblog: android_closeEventTagMap pointer check

Followup to c/278768 where failure to open tag map can
result in segment violation in android_closeEventTagMap.
Add check for pointer value = -1 to bypass call to close.

Test: gTest liblog-unit-tests, logcat-unit-tests & logd-unit-tests
Bug: 30963384
Bug: 31456426
Change-Id: I20547e55d7e29682fde4538bc1fc6d83b4535d49