pulsesrc: fix checking for invalid stream index

PA_INVALID_INDEX, the default value, is unfortunately !0.

Setting the volume before the stream is created will put the ring
buffer in error state. Unfortunately, that's what spice-gtk does.
6 jobs for pulse in 26 minutes and 27 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Preparation
passed #74551
gst indent

00:00:32

passed #74550
manifest

00:00:19

 
  Build
passed #74552
build fedora x86_64

00:03:08

 
  Test
passed #74553
check fedora

00:15:54

 
  Full Builds
passed #74554
build android arm64 api28

00:02:48

passed #74555
build cerbero fedora x86_64

00:06:51