ostrace: Fix memory leak (#1677)

Thanks to @mickelluchko for reporting

src/ostrace.c

diff --git a/src/ostrace.c b/src/ostrace.c
index 68eb6bf..c73ee7f 100644
--- a/src/ostrace.c
+++ b/src/ostrace.c
@@ -251,6 +251,7 @@
 			break;
 		}
 		oswt->cbfunc(buf, received, oswt->user_data);
+		free(buf);
 	}
 
 	if (oswt) {