Ignore the return value of g_string_append()

Calling “x = g_string_append (x, y)” is equivalent to “g_string (x, y)”. Or take the return value into account, when this leads to shorter code.

In camel_content_disposition_format: skip a realloc() call.

In camel-sasl-ntlm.c: skip a g_string_append_len() call.