fopen: remove unnecessary assignment

[CWE-1164] V1048: The '* tempname' variable was assigned the same value.

Ref: https://pvs-studio.com/en/docs/warnings/v1048/

Closes https://github.com/curl/curl/pull/10398

lib/fopen.c

@@ -106,7 +106,6 @@ fail:
 
   free(tempstore);
 
-  *tempname = NULL;
   return result;
 }