Fix: prevent crash when stdoutRing or stderrRing is undefined

[decompi]

This pull request fixes a bug that causes fluent-ffmpeg to crash when stdoutRing or stderrRing are undefined which happens when FFmpeg exits early or fails to spawn properly.

Problem:
In some rare cases (see #1316 ), the following error occurs:

TypeError: Cannot read properties of undefined (reading 'get')
    at emitEnd (.../lib/processor.js:543:37)

This indicates stdoutRing or stderrRing may have not been initialized in time

My Patch:

• Adds a null-safe check using stdoutRing?.get?.() and fallback to null if undefined.
• Logs a warning when either ring is unexpectedly missing

Changed Lines:

• Line 470 (timeout handler)
• Line 502(output stream err handler)
• Line 550 (final endCB cleanup)