Keep Coverity Scan happy

Coverity Scan (https://scan.coverity.com/projects/trader) complains that
a high-impact defect is present in the scramble() function: "String not
null terminated" -- even though b64encode() does give a NUL-termininated
buffer.

src/utils.c

@@ -929,6 +929,7 @@ char *scramble (char *restrict dest, const char *restrict src,
 	// Place checksum in front of Base64 string
 	snprintf(chksumbuf, SCRAMBLE_CHKSUM_LEN + 1, "%03x", chksum);
 	memcpy(dest, chksumbuf, SCRAMBLE_CHKSUM_LEN);
+	dest[size - 1] = '\0';		// Keep Coverity Scan happy...
 
 	free(xorbuf);
     }