mac Plumb: skip loading non-existent .bashrc

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/5434052

CONTRIBUTORS

@@ -28,6 +28,7 @@ Michael Teichgräber <mt@ib.wmipf.de>
 Nikolai Saoukh <nikolai.saoukh@gmail.com>
 Peter Saveliev <svinota.saveliev@gmail.com>
 Richard Miller <millerresearch@gmail.com>
+Rob Kroeger <robkroeger@gmail.com>
 Rob Pike <robpike@gmail.com>
 Russ Cox <rsc@swtch.com>
 Sean McKean <smckean83@gmail.com>

mac/Plumb.app/Contents/MacOS/plumb

@@ -1,6 +1,8 @@
 #!/bin/bash
 
-. ~/.bashrc
+if [ -e ~/.bashrc ] ; then
+	. ~/.bashrc
+fi
 PLAN9=${PLAN9:-/usr/local/plan9}
 
 bin=$PLAN9/bin