libxl: don't leak memory in libxl__poller_get failure case

Coverity-ID: 1055894
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
(cherry picked from commit 1edd6d8da3)

tools/libxl/libxl_event.c

@@ -1340,7 +1340,10 @@ libxl__poller *libxl__poller_get(libxl_ctx *ctx)
     memset(p, 0, sizeof(*p));
 
     rc = libxl__poller_init(ctx, p);
-    if (rc) return NULL;
+    if (rc) {
+        free(p);
+        return NULL;
+    }
 
     return p;
 }