Fix gmime assertions being triggered by invalid addresses

[ibuclaw]

In the console, I am frequently seeing the following assertions being triggered.

(astroid:1774218): gmime-CRITICAL **: 18:25:20.896: internet_address_list_length: assertion 'IS_INTERNET_ADDRESS_LIST (list)' failed
(astroid:1774218): gmime-CRITICAL **: 18:25:20.896: internet_address_list_length: assertion 'IS_INTERNET_ADDRESS_LIST (list)' failed
[18:25:20] [0x00007fffc2d6c6c0] [M] [error] address: no address in string.

Silence this by checking IS_INTERNET_ADDRESS_LIST before calling internet_address_list_length.

[gauteh]

Thanks. Merged.