[audio-utils] Never let strncpy touch the end of the buffer

It's considered best practice never to give strncpy the whole
buffer, and some compilers will warn about doing so.  In this
case the code has no bug since it always terminates the buffer
correctly after strncpy might have written a nonzero byte at the
end, but the compiler doesn't notice that.

Change-Id: I6c4aa72d1d33bfcd2e05fcb04e607fd0e4bdaa54
diff --git a/zircon/system/ulib/audio-utils/audio-device-stream.cpp b/zircon/system/ulib/audio-utils/audio-device-stream.cpp
index d2ac4db..4ee168e 100644
--- a/zircon/system/ulib/audio-utils/audio-device-stream.cpp
+++ b/zircon/system/ulib/audio-utils/audio-device-stream.cpp
@@ -98,7 +98,7 @@
 
 AudioDeviceStream::AudioDeviceStream(bool input, const char* dev_path)
     : input_(input) {
-    strncpy(name_, dev_path, sizeof(name_));
+    strncpy(name_, dev_path, sizeof(name_) - 1);
     name_[sizeof(name_) - 1] = 0;
 }