Link to correct files directory

Line 951 of the visualize_backtrace.module has the "/files" directory hardcoded, and in Drupal 6 the default files directory is now located at: "sites/default/files"

// There is an extra space at the end of $unix_base_path that needs to be deleted.
     $dot_file[$section_number] = trim($unix_base_path) ."/files/". $trace_number ."_section". $section_number .".dot";

An admin workaround fix is to create a files directory at /files and to point to it at example.com/admin/settings/file-system
Then the *.dot files can be properly saved and processed.

The code fix should be to replace the ."/files/" with the proper system variable.

The error comes up will look something like this:

warning: fopen(/Library/WebServer/Documents/d62_5/files/1213167960_section0.dot) [function.fopen]: failed to open stream: No such file or directory in /Library/WebServer/Documents/d62_5/sites/all/modules/visualize_backtrace/visualize_backtrace.module on line 952.
warning: fwrite(): supplied argument is not a valid stream resource in /Library/WebServer/Documents/d62_5/sites/all/modules/visualize_backtrace/visualize_backtrace.module on line 953.
warning: fclose(): supplied argument is not a valid stream resource in /Library/WebServer/Documents/d62_5/sites/all/modules/visualize_backtrace/visualize_backtrace.module on line 954.
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