How to correctly check the value of boolean in D7?
The error you're getting simply means that 

$content['field_hide_1st_line_of_the_addre']['und']


hasn't been set. It might be that the 'und' key is missing, or it might be that $content['field_hide_1st_line_of_the_addre'] simply hasn't been set at all. This can happen if you've added a new field to a content type that already has existing nodes (and probably in other situations too).

To remove the notice, you can just wrap your code in another condition that ensures the array element you're accessing exists. For example:

&lt;?php 
  if (isset($content['field_hide_1st_line_of_the_addre'][LANGUAGE_NONE][0]['value'])) :
    if ($content['field_hide_1st_line_of_the_addre'][LANGUAGE_NONE][0]['value'] == 0) : 
      print render($content['field_images']['#object']-&gt;field_location['und'][0]['street']) . t(', '); 
    endif;
  endif; 
?&gt;


I've taken the liberty of replacing 'und' with the constant equivalent (and cleared up a couple of other very minor issues), it's better practice. Even better practice would be to extract this logic out of the template file completely, and instead implement it in a preprocess function.