FormAdd: precaution against memdup() of NULL pointer

Coverity CID 252518. This function is in general far too complicated for
its own good and really should be broken down into several smaller
funcitons instead - but I'm adding this protection here now since it
seems there's a risk the code flow can end up here and dereference a
NULL pointer.

lib/formdata.c

@@ -689,7 +689,7 @@ CURLFORMcode FormAdd(struct curl_httppost **httppost,
         }
         if(!(form->flags & (HTTPPOST_FILENAME | HTTPPOST_READFILE |
                             HTTPPOST_PTRCONTENTS | HTTPPOST_PTRBUFFER |
-                            HTTPPOST_CALLBACK)) ) {
+                            HTTPPOST_CALLBACK)) && form->value) {
           /* copy value (without strdup; possibly contains null characters) */
           form->value = memdup(form->value, form->contentslength);
           if(!form->value) {