ncclDebugLog: fix for log payload larger than buf

[danielahlin]

The logging function ncclDebugLog does not properly check the return value of the vsnprintf call (or, less importantly, the snprintf calls). Consequenly, when the length of the logging header and the logging message payload is larger than buf, then :

1. Log messages are incorrect.
2. buffer[len++] = '\n' writes out of bounds.
3. fwrite(buffer, 1, len, ncclDebugFile) reads out of bounds.

[sjeaugey]

Is this a theoretical issue, or did you encounter cases where we would print a line so long it would exceed the 1024 chars in buffer?

[danielahlin]

I found it when developing #826.

I don't remember right now what I printed to the log, but probably the value of some environment variable.

[nzmsv]

I also ran into this and wrote a patch before noticing this ticket (#1215)