logcat: continue where we left off

Issue introduced as part of new logcatd functionality in
commit f3555d9427425c2cba9600ceffb49305c440aa4a

Faulty logic, add a gTest to confirm.

Bug: 19608716
Change-Id: Ic1b97def25e03e69faae4398a3dff2ff0f88545e
diff --git a/logcat/logcat.cpp b/logcat/logcat.cpp
index 9440b68..54cbc20 100644
--- a/logcat/logcat.cpp
+++ b/logcat/logcat.cpp
@@ -656,7 +656,7 @@
             break;
 
             case 'f':
-                if ((tail_time == log_time::EPOCH) && (tail_lines != 0)) {
+                if ((tail_time == log_time::EPOCH) && (tail_lines == 0)) {
                     tail_time = lastLogTime(optarg);
                 }
                 // redirect output to a file