Update pcm.c

avail will be wrong when avail is same with pcm->boundary.
diff --git a/pcm.c b/pcm.c
index ca3eb3e..66c6635 100644
--- a/pcm.c
+++ b/pcm.c
@@ -768,7 +768,7 @@
 
     if (avail < 0)
         avail += pcm->boundary;
-    else if (avail > (int)pcm->boundary)
+    else if (avail >= (int)pcm->boundary)
         avail -= pcm->boundary;
 
     return avail;