urlapi: accept :: as a valid IPv6 address

Text 1560 is extended to verify.

Reported-by: Pavel Volgarev
Fixes #5344
Closes #5351

lib/urlapi.c

@@ -606,7 +606,7 @@ static CURLUcode hostname_check(struct Curl_URL *u, char *hostname)
     char dest[16]; /* fits a binary IPv6 address */
 #endif
     const char *l = "0123456789abcdefABCDEF:.";
-    if(hlen < 5) /* '[::1]' is the shortest possible valid string */
+    if(hlen < 4) /* '[::]' is the shortest possible valid string */
       return CURLUE_MALFORMED_INPUT;
     hostname++;
     hlen -= 2;

tests/libtest/lib1560.c

@@ -129,6 +129,15 @@ struct querycase {
 };
 
 static struct testcase get_parts_list[] ={
+  {"[::1]",
+   "http | [11] | [12] | [13] | [::1] | [15] | / | [16] | [17]",
+   CURLU_GUESS_SCHEME, 0, CURLUE_OK },
+  {"[::]",
+   "http | [11] | [12] | [13] | [::] | [15] | / | [16] | [17]",
+   CURLU_GUESS_SCHEME, 0, CURLUE_OK },
+  {"https://[::1]",
+   "https | [11] | [12] | [13] | [::1] | [15] | / | [16] | [17]",
+   0, 0, CURLUE_OK },
   {"user:moo@ftp.example.com/color/#green?no-black",
    "ftp | user | moo | [13] | ftp.example.com | [15] | /color/ | [16] | "
    "green?no-black",