allow 4 arguments to bpsk_costas_loop_cc

[ddrown]

without this patch, bpsk_costas_loop_cc crashes when given 4 arguments:

(gdb) run bpsk_costas_loop_cc 0.003 0.6 --output_nco
Starting program: /home/abob/csdr/csdr bpsk_costas_loop_cc 0.003 0.6 --output_nco

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b7e75a in __strcmp_ssse3 () from /lib64/libc.so.6
(gdb) bt
#0  0x00007ffff7b7e75a in __strcmp_ssse3 () from /lib64/libc.so.6
#1  0x000000000040b416 in main (argc=5, argv=0x7fffffffe388) at csdr.c:2844