found segfault in S3fsMultiCurl::MultiRead(void)

Detailed description of observed behavior:

I found segfault in S3fsMultiCurl::MultiRead(void) in "s3fs-1.74"

As you know readdir_multi_head() function registered 
multi_head_retry_callback() at RetryCallback to use retry callback.

And in "multi_head_retry_callback(2121): Over retry count(3) limit" case, 
multi_head_retry_callback() return NULL.

by following code of in S3fsMultiCurl::MultiRead(void)

// For retry
  if(RetryCallback){
     retrycurl = RetryCallback(s3fscurl);
     cMap_all[retrycurl->hCurl] = retrycurl;
  }

retrycurl is NULL, ...........

So I changed like following code
   // For retry
  if(RetryCallback){
     retrycurl = RetryCallback(s3fscurl);
     if(retrycurl)
         cMap_all[retrycurl->hCurl] = retrycurl;
     else
         return -1;
  }

Please check it for me

Best regards,
- BONG

What steps will reproduce the problem - please be very specific and
detailed. (if the developers cannot reproduce the issue, then it is
unlikely a fix will be found)?

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Version of s3fs being used (s3fs --version):

Version of fuse being used (pkg-config --modversion fuse):

System information (uname -a):

Distro (cat /etc/issue):

s3fs command line used (if applicable):

/etc/fstab entry (if applicable):

s3fs syslog messages (grep s3fs /var/log/syslog):

Original issue reported on code.google.com by b...@gscdn.com on 28 Jul 2014 at 6:16

yangljun / s3fs

found segfault in S3fsMultiCurl::MultiRead(void) #437