Fix incorrect checks for the fwrite() return value

! fwrite() could trigger false-positives as it is possible for it to return 0
instead of boolean FALSE. (issue #2822)

Also removed an unnecessary log level check that caused an extra space to be inserted
for the INFO level. (proposed in PR #2821)
diff --git a/system/core/Log.php b/system/core/Log.php
index b2327b8..ff3c635 100644
--- a/system/core/Log.php
+++ b/system/core/Log.php
@@ -175,7 +175,7 @@
 			return FALSE;
 		}
 
-		$message .= $level.' '.($level === 'INFO' ? ' -' : '-').' '.date($this->_date_fmt).' --> '.$msg."\n";
+		$message .= $level.' - '.date($this->_date_fmt).' --> '.$msg."\n";
 
 		flock($fp, LOCK_EX);
 		fwrite($fp, $message);