base32_decode: avoid shifting into the sign bit

While this won't do harm on systems that do 2's completement,
it triggers the compilers' undefined-behavior sanitizer and
fixes sanitizer error such as the one below (where the 1694... will
vary) and is distracting while debugging.

dnssec.c:1427:12: runtime error: left shift of 1694604366 by 1 places
cannot be represented in type 'int'

src/dnssec.c

@@ -1423,7 +1423,10 @@ static int base32_decode(char *in, unsigned char *out)
 	    oc |= 1;
 	  mask = mask >> 1;
 	  if (((++on) & 7) == 0)
-	    *p++ = oc;
+	    {
+	      *p++ = oc;
+	      oc = 0;
+	    }
 	  oc = oc << 1;
 	}
     }