Prevent buffer overflow when formatting the error
Prevent buffer overflow when formatting the error

strncat might copy n+1 bytes (n bytes from the source plus a terminating nul byte).
Also strncat appends after the first found nul byte. But all we pass is
a buffer we might not have zeroed out already.

Closes #380

redis/hiredis,charsyam/hiredis,thomaslee/hiredis,owent-contrib/hiredis,thomaslee/hiredis,owent-contrib/hiredis,redis/hiredis,jinguoli/hiredis,redis/hiredis,jinguoli/hiredis,jinguoli/hiredis,charsyam/hiredis