[c] Fix Atlas line length calculation, closes #1925

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.19)
+cmake_minimum_required(VERSION 3.17)
 project(spine)
 
 if(MSVC)

spine-c/spine-c/src/spine/Atlas.c

@@ -220,7 +220,7 @@ static SimpleString *ai_readLine(AtlasInput *self) {
 	self->line.end = self->index;
 	if (self->index != self->end) self->index++;
 	self->line = *ss_trim(&self->line);
-	self->line.length = self->end - self->start;
+	self->line.length = self->line.end - self->line.start;
 	return &self->line;
 }