ShellPkg/HttpDynamicCommand: Fix possible uninitialized use

ShellPkg/HttpDynamicCommand: Fix possible uninitialized use

`Status` can be used uninitialized:

    /* Evaluates to FALSE */
    if (ShellGetExecutionBreakFlag ()) {
        Status = EFI_ABORTED;
        break;
    }

    /* Evaluates to FALSE */
    if (!Context->ContentDownloaded && !Context->ResponseToken.Event) {
        Status = ...;
        ASSERT_EFI_ERROR (Status);
    } else {
        ResponseMessage.Data.Response = NULL;
    }

    /* UNINITIALIZED USE */
    if (EFI_ERROR (Status)) {
        break;
    }

Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Sergei Dmitrouk <sergei@posteo.net>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>