qemu: agent: fix uninitialized var case in qemuAgentGetFSInfo

In case of 0 filesystems *info is not set while according
to virDomainGetFSInfo contract user should call free on it even
in case of 0 filesystems. Thus we need to properly set
it. NULL will be enough as free eats NULLs ok.

src/qemu/qemu_agent.c

@@ -1872,6 +1872,7 @@ qemuAgentGetFSInfo(qemuAgentPtr mon, virDomainFSInfoPtr **info,
     ndata = virJSONValueArraySize(data);
     if (!ndata) {
         ret = 0;
+        *info = NULL;
         goto cleanup;
     }
     if (VIR_ALLOC_N(info_ret, ndata) < 0)