curl_easy_escape: deny negative string lengths as input

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;