logd: propagate ::log status

Add a return value for the ::log() methods, this allows
us to optimize the wakeup for the readers to only occur
when the log message is actually placed.

This is for a future where we may dedupe identical log
messages, filter out log messages, and certainly if we
filter the messages out with an internal logd check of
__android_log_is_loggable().

Change-Id: I763b2a7c29502ab7fa0a5d5022c7b60244fcfde4
diff --git a/logd/main.cpp b/logd/main.cpp
index 4aa38b3..eb29596 100644
--- a/logd/main.cpp
+++ b/logd/main.cpp
@@ -367,12 +367,12 @@
 
             int rc = klogctl(KLOG_READ_ALL, buf, len);
 
-            buf[len - 1] = '\0';
+            if (rc >= 0) {
+                buf[len - 1] = '\0';
 
-            for(char *ptr, *tok = buf;
-                    (rc >= 0) && ((tok = strtok_r(tok, "\r\n", &ptr)));
-                    tok = NULL) {
-                rc = al->log(tok);
+                for (char *ptr, *tok = buf; (tok = strtok_r(tok, "\r\n", &ptr)); tok = NULL) {
+                    al->log(tok);
+                }
             }
         }