AddFormData: prevent only directories from being posted

Commit f4cc54cb47 (shipped as part of the 7.29.0 release) was a
bug fix that introduced a regression in that while trying to avoid
allowing directory names, it also forbade "special" files like character
devices and more. like "/dev/null" as was used by Oliver who reported
this regression.

Reported by: Oliver Gondža
Bug: http://curl.haxx.se/mail/archive-2013-02/0040.html

lib/formdata.c

@@ -831,7 +831,7 @@ static CURLcode AddFormData(struct FormData **formp,
          file */
       if(!strequal("-", newform->line)) {
         struct_stat file;
-        if(!stat(newform->line, &file) && S_ISREG(file.st_mode))
+        if(!stat(newform->line, &file) && !S_ISDIR(file.st_mode))
           *size += file.st_size;
         else
           return CURLE_BAD_FUNCTION_ARGUMENT;