Properly handle cases where a stream can't be written to. (issue #9757 reported by junky)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@67924 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/file.c

@@ -956,6 +956,8 @@ struct ast_filestream *ast_writefile(const char *filename, const char *type, con
 				}
 				if (fs)
 					ast_free(fs);
+				fs = NULL;
+				continue;
 			}
 			fs->trans = NULL;
 			fs->fmt = f;