URL parse: reject numerical IPv6 addresses outside brackets

Roman Mamedov spotted (in
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=670126) that curl would
not complain when given a URL with an IPv6 numerical address without
brackets. It would simply cut off the last ":[hex]" part and thus not
work correctly.

That's a URL using an illegal syntax and now libcurl will instead return
a clear error code and error message detailing the error.

The above mentioned bug report claims this to be a regression but
libcurl does not guarantee functionality when given URLs that aren't
following the URL spec (RFC3986 mostly). I consider the fact that it
used to handle this differently a mere coincidence.

lib/url.c

@@ -105,6 +105,7 @@ int curl_win32_idn_to_ascii(const char *in, char **out);
 #include "rawstr.h"
 #include "warnless.h"
 #include "non-ascii.h"
+#include "inet_pton.h"
 
 /* And now for the protocols */
 #include "ftp.h"
@@ -4495,8 +4496,19 @@ static CURLcode parse_remote_port(struct SessionHandle *data,
         portptr = NULL; /* no port number available */
     }
   }
-  else
+  else {
+#ifdef ENABLE_IPV6
+    struct in6_addr in6;
+    if(Curl_inet_pton(AF_INET6, conn->host.name, &in6) > 0) {
+      /* This is a numerical IPv6 address, meaning this is a wrongly formatted
+         URL */
+      failf(data, "IPv6 numerical address used in URL without brackets");
+      return CURLE_URL_MALFORMAT;
+    }
+#endif
+
     portptr = strrchr(conn->host.name, ':');
+  }
 
   if(data->set.use_port && data->state.allow_port) {
     /* if set, we use this and ignore the port possibly given in the URL */