curl_easy_escape: deny negative string lengths as input

CVE-2016-7167

Bug: https://curl.haxx.se/docs/adv_20160914.html

lib/escape.c

@@ -78,15 +78,21 @@ char *curl_unescape(const char *string, int length)
 char *curl_easy_escape(struct Curl_easy *data, const char *string,
                        int inlength)
 {
-  size_t alloc = (inlength?(size_t)inlength:strlen(string))+1;
+  size_t alloc;
   char *ns;
   char *testing_ptr = NULL;
   unsigned char in; /* we need to treat the characters unsigned */
-  size_t newlen = alloc;
+  size_t newlen;
   size_t strindex=0;
   size_t length;
   CURLcode result;
 
+  if(inlength < 0)
+    return NULL;
+
+  alloc = (inlength?(size_t)inlength:strlen(string))+1;
+  newlen = alloc;
+
   ns = malloc(alloc);
   if(!ns)
     return NULL;