Logger hotfix: Allocate buffer on logging

This enables to use the RAM normally consumed by the log buffer to be used for calibration and other memory-intense tasks.
These run typically only disarmed when logging is not enabled.

src/modules/logger/log_writer_file.cpp

@@ -60,13 +60,7 @@ LogWriterFile::LogWriterFile(size_t buffer_size) :
 
 bool LogWriterFile::init()
 {
-	if (_buffer) {
-		return true;
-	}
-
-	_buffer = new uint8_t[_buffer_size];
-
-	return _buffer;
+	return true;
 }
 
 LogWriterFile::~LogWriterFile()
@@ -91,6 +85,17 @@ void LogWriterFile::start_log(const char *filename)
 		return;
 
 	} else {
+
+		if (_buffer == nullptr) {
+			_buffer = new uint8_t[_buffer_size];
+
+			if (_buffer == nullptr) {
+				PX4_ERR("Can't create log buffer");
+				_should_run = false;
+				return;
+			}
+		}
+
 		PX4_INFO("Opened log file: %s", filename);
 		_should_run = true;
 		_running = true;