message: Ensure that the "debug" field of error/warning/info messages is valid UTF-8

message: Ensure that the "debug" field of error/warning/info messages is valid UTF-8

The caller might pass arbitrary data here that caused the error, and
trying to set invalid UTF-8 in a GstStructure causes it to be not set at
all. Later when trying to parse it, the field will not exist and the
return value will point to invalid memory. Prevent this by storing NULL
instead.

Also print a g_warning(), the caller should never ever do this to begin
with.