SVCB fromwire should reject zero length ALPN elements

Fromtext already prevents these.

(lldb) parray 12 rdata->data
(unsigned char *) $5 = 0x0000000104009201 "  " {
  (unsigned char) [0] = ' '
  (unsigned char) [1] = ' '
  (unsigned char) [2] = '\0'
  (unsigned char) [3] = '\0'
  (unsigned char) [4] = '\x01'
  (unsigned char) [5] = '\0'
  (unsigned char) [6] = '\x05'
  (unsigned char) [7] = '\0'
  (unsigned char) [8] = '\0'
  (unsigned char) [9] = '\0'
  (unsigned char) [10] = '\0'
  (unsigned char) [11] = '\0'
}
(lldb)