check_http segfaults with "-a x" option

To reproduce:

$ /usr/lib/nagios/plugins/check_http google.com
HTTP OK HTTP/1.0 200 OK - 6094 bytes in 0.134 seconds |time=0.133703s;;;0.000000 size=6094B;;;0
$ /usr/lib/nagios/plugins/check_http google.com -a x
Segmentation fault (core dumped)
$ /usr/lib/nagios/plugins/check_http google.com -a xx
HTTP OK HTTP/1.0 200 OK - 6084 bytes in 0.151 seconds |time=0.150939s;;;0.000000 size=6084B;;;0

Backtrace follows.

$ gdb check_http
[...]
(gdb) run google.com -a x
[...]
Program received signal SIGSEGV, Segmentation fault.
0x0804b219 in check_http () at check_http.c:469
469 buf[i++] = base64_table[bin[j] >> 2];
(gdb) bt
#0 0x0804b219 in check_http () at check_http.c:469
#1 0x0804e3f0 in main (argc=Cannot access memory at address 0x17f1c
) at check_http.c:160

It appears that a one-character argument to "-a" causes a segfault, while more than one character averts this. A one-character argument to "-a" isn't a conceivably valid option, but the program still shouldn't crash.

This is replicable with nagios-plugins 1.4.8-2.1ubuntu1.1 on Gutsy, and is still present in 1.4.11 downloaded from nagiosplugins.org.