[PATCH] Fix segfault in fast-export

Hi all,

I'm working on a C++ static analyzer (Vigilant Sentry), and git is one of my test subjects. In git-1.6.6, I found a crash in the fast-export command:

The problem is in builtin-fast-export.c, function export_marks:

    f = fopen(file, "w");
    if (!f)
        error("Unable to open marks file %s for writing.", file);
   
    for (i = 0; i < idnums.size; i++) {
        if (deco->base && deco->base->type == 1) {
            mark = ptr_to_mark(deco->decoration);
            if (fprintf(f, ":%"PRIu32" %s\n", mark,
                sha1_to_hex(deco->base->sha1)) < 0) {
                e = 1;
                break;
            }
        }
        deco++;
    }
   
    e |= ferror(f);
    e |= fclose(f);

If fopen() fails, the error message is printed, but the function
doesn't exit.  The subsequent calls to fprintf and/or ferror will
fail because f is NULL.  A simple way to reproduce is to export
to a path you don't have write access to:
   
    $ git fast-export --export-marks=/foo
    error: Unable to open marks file /foo for writing.
    Segmentation fault (core dumped)

I've attached a trivial patch that calls die_errno instead of error, so the program exits if f is NULL.

Regards, Mike