Bugfixes for rtp_analyzer.py tool.

2 fixes: When running tool on log with no packet losses, the tool no
longer crashes. When providing relative path to log, the tool no
longer searches in out/target, but instead in current directory.

NOTRY=True

Review-Url: https://codereview.webrtc.org/2291473003
Cr-Commit-Position: refs/heads/master@{#13954}

webrtc/tools/py_event_log_analyzer/rtp_analyzer.sh

@@ -7,10 +7,11 @@
 #  in the file PATENTS.  All contributing project authors may
 #  be found in the AUTHORS file in the root of the source tree.
 set -e
+WORKING_DIR=$(pwd)
 cd $(dirname $0)
 PYTHONPATH="../../third_party/protobuf/python/"
 if [ -z ${PYTHON_EXECUTABLE+x} ]
 then
     PYTHON_EXECUTABLE=python3
 fi
-exec $PYTHON_EXECUTABLE "rtp_analyzer.py" $@
+exec $PYTHON_EXECUTABLE "rtp_analyzer.py" $@ --working_dir $WORKING_DIR