to set the size of a GArray, and similarly GByteArray.

Simply assigning a value to array->len works when shortening an array
with no free-function, but the doc says:
"To set the size of an array, use g_array_set_size()."

modified:   libbalsa/mailbox.c
modified:   libbalsa/mailbox_imap.c
modified:   libbalsa/mailbox_mbox.c
modified:   libbalsa/mailbox_mh.c
modified:   libbalsa/message.c