doh: fix doh_encode's error return value

- Return 0 on doh_encode error. This makes the values returned on error
  by doh_encode() consistent with the post-invocation test.

Prior to this change doh_encode, which returns a size_t of bytes
written on success, erroneously returned DOHcodes on error
(eg DOH_TOO_SMALL_BUFFER (4), ie 4 bytes written, which was wrong).

Fixes #40
Closes #41

Author: niallor

doh.c

@@ -256,7 +256,7 @@ static size_t doh_encode(const char *host,
   const char *hostp = host;
 
   if(len < (12 + hostlen + 6))
-    return DOH_TOO_SMALL_BUFFER;
+    return 0;
 
   *dnsp++ = 0; /* 16 bit id */
   *dnsp++ = 0;
@@ -284,7 +284,7 @@ static size_t doh_encode(const char *host,
       labellen = strlen(hostp);
     if(labellen > 63)
       /* too long label, error out */
-      return DOH_DNS_BAD_LABEL;
+      return 0;
     *dnsp++ = (unsigned char)labellen;
     memcpy(dnsp, hostp, labellen);
     dnsp += labellen;