MdeModulePkg/PlatformDriOverrideDxe: Fix overflow condition check
    
    Code mistake, VariableIndex is smaller normally than buffer+buffersize
    so should not break loop.
    
    Signed-off-by: Walon Li <walon.li@hpe.com>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>