logger: remove _log_buffer from Logger, initialize it in the writer instead

it's not used in the logger, so don't store it there. It is accessed via
LogWriter::write.
This also makes sure the buffer size is >= _min_write_chunk and handles
allocation failure properly.

src/modules/logger/log_writer.cpp

@@ -2,15 +2,15 @@
 #include <fcntl.h>
 #include <string.h>
 
+#include <mathlib/mathlib.h>
+
 namespace px4
 {
 namespace logger
 {
 
-LogWriter::LogWriter(uint8_t *buffer, size_t buffer_size) :
-	_buffer(buffer),
-	_buffer_size(buffer_size),
-	_min_write_chunk(4096)
+LogWriter::LogWriter(size_t buffer_size) :
+	_buffer_size(math::max(buffer_size, _min_write_chunk))
 {
 	pthread_mutex_init(&_mtx, nullptr);
 	pthread_cond_init(&_cv, nullptr);
@@ -19,6 +19,24 @@ LogWriter::LogWriter(uint8_t *buffer, size_t buffer_size) :
 	perf_fsync = perf_alloc(PC_ELAPSED, "sd fsync");
 }
 
+bool LogWriter::init()
+{
+	if (_buffer) {
+		return true;
+	}
+
+	_buffer = new uint8_t[_buffer_size];
+
+	return _buffer;
+}
+
+LogWriter::~LogWriter()
+{
+	if (_buffer) {
+		delete[] _buffer;
+	}
+}
+
 void LogWriter::start_log(const char *filename)
 {
 	::strncpy(_filename, filename, sizeof(_filename));