🐛 Fix segfaults in writer and gstshmdatasrc

Fixing two separate segfaults:

1. In the gst_shmdata_src_on_server_connect function from gstshmdatasrc.c, sometimes user_data cannot be cast into a GstShmdataSrc pointer, which results in the function trying to access the caps property of a NULL pointer and causing a segfault.
2. In the copy_to_shm function from writer.cpp, sometimes the function tries to lock a semaphore that doesn't exist or isn't valid.