fix: Emit error message for an unknown i_error, rather than an empty string. (#1030)

Suggested by @david-BarOn.

Towards #1022.

src/iperf_error.c

@@ -424,6 +424,10 @@ iperf_strerror(int int_errno)
 	case IETOTALRATE:
 	    snprintf(errstr, len, "total required bandwidth is larger than server limit");
             break;
+	default:
+	    snprintf(errstr, len, "int_errno=%d", int_errno);
+	    perr = 1;
+	    break;
     }
 
     /* Append the result of strerror() or gai_strerror() if appropriate */