url: fix buffer overwrite with file protocol (CVE-2017-9502)

Bug: https://github.com/curl/curl/issues/1540
Advisory: https://curl.haxx.se/docs/adv_20170614.html

Assisted-by: Ray Satiro
Reported-by: Marcel Raad

lib/url.c

@@ -4466,6 +4466,7 @@ static CURLcode parseurlandfillconn(struct Curl_easy *data,
 #endif
 
     protop = "file"; /* protocol string */
+    *prot_missing = !url_has_scheme;
   }
   else {
     /* clear path */
@@ -4629,14 +4630,30 @@ static CURLcode parseurlandfillconn(struct Curl_easy *data,
 
     size_t plen = strlen(path); /* new path, should be 1 byte longer than
                                    the original */
-    size_t urllen = strlen(data->change.url); /* original URL length */
-
     size_t prefixlen = strlen(conn->host.name);
 
-    if(!*prot_missing)
-      prefixlen += strlen(protop) + strlen("://");
+    if(!*prot_missing) {
+      size_t protolen = strlen(protop);
 
-    reurl = malloc(urllen + 2); /* 2 for zerobyte + slash */
+      if(curl_strnequal(protop, data->change.url, protolen))
+        prefixlen += protolen;
+      else {
+        failf(data, "<url> malformed");
+        return CURLE_URL_MALFORMAT;
+      }
+
+      if(curl_strnequal("://", &data->change.url[protolen], 3))
+        prefixlen += 3;
+      /* only file: is allowed to omit one or both slashes */
+      else if(curl_strnequal("file:", data->change.url, 5))
+        prefixlen += 1 + (data->change.url[5] == '/');
+      else {
+        failf(data, "<url> malformed");
+        return CURLE_URL_MALFORMAT;
+      }
+    }
+
+    reurl = malloc(prefixlen + plen + 1);
     if(!reurl)
       return CURLE_OUT_OF_MEMORY;