NetworkPkg/Ip6Dxe: Validate source data record length
    
    REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2273
    
    Ip6ConfigReadConfigData() reads configuration data from a UEFI variable
    and copies the data to another buffer. This change checks that the
    length
    of the data record being copied does not exceed the size of the source
    UEFI variable data buffer.
    
    If the size is exceeded, this change follows existing logic to treat the
    variable as corrupted and deletes the variable so it will be set again.
    
    Cc: Siyuan Fu <siyuan.fu@intel.com>
    Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
    Cc: Jiaxin Wu <jiaxin.wu@intel.com>
    Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
    Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
    Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>