Handle NULL strings like g_strdup. s/g_strconcat_a/g_strconcat3_a/ to

Handle NULL strings like g_strdup. s/g_strconcat_a/g_strconcat3_a/ to

        * glib.h:
        (g_strdup_a, g_strndup_a): Handle NULL strings like g_strdup.
        s/g_strconcat_a/g_strconcat3_a/ to reflect fixed number of args

        * testglib.c:
        Added g_strdup, g_strconcat checks.
        Added str==NULL checks for alloca string macros.
        s/g_strconcat_a/g_strconcat3_a/