More strict error check of mg_init_library()
More strict error check of mg_init_library()

(Commit by xtne6f, merge conflict resolved by bel2125)

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver