Fix E_ALL and E_STRICT warnings

The module currently generates quite a lot of notices/warnings on the E_ALL and E_STRICT error levels.

The following types of notices are generated when viewing the mapping screen of a feed node; there are usually 100+ of these notices (see attached screenshot):

    * notice: Undefined variable: output in modules/feedapi_mapper/feedapi_mapper.module on line 112.
    * notice: Undefined property: stdClass::$feedapi in modules/feedapi_mapper/feedapi_mapper.module on line 50.
    * notice: Trying to get property of non-object in modules/feedapi_mapper/feedapi_mapper.module on line 50.
    * notice: Undefined property: stdClass::$name in /Users/arto/Sites/d6.local/modules/node/node.pages.inc on line 181.
    * notice: Undefined variable: merged in modules/feedapi_mapper/feedapi_mapper.module on line 322.
    * notice: Undefined index: geo_lat in modules/feedapi_mapper/feedapi_mapper.module on line 452.
    * notice: Undefined index: geo_long in modules/feedapi_mapper/feedapi_mapper.module on line 443.
    * notice: Undefined variable: output in modules/feedapi_mapper/feedapi_mapper.module on line 580.
    * notice: Undefined index: a:1:{i:0;s:5:"title";} in modules/feedapi_mapper/feedapi_mapper.module on line 192.

The following types of notices are generated when refreshing a feed node:

    * notice: Undefined offset: 2 in modules/feedapi_mapper/feedapi_mapper.module on line 78.
    * notice: Undefined property: stdClass::$field_url in modules/feedapi_mapper/mappers/feedapi_mapper_content.inc on line 37.
    * notice: Undefined index: geo_lat in modules/feedapi_mapper/feedapi_mapper.module on line 93.

I've fixed all the problems that appeared on my local test installation and have rolled the fixes up into the attached patch. These are very minor changes, basically just needing somewhat more defensive coding in places, such as making sure to always explicitly use isset() for array subscripts (as Drupal core does, too).