Fix T39941: Freestyle: Rendered preview crashes when non-primary render layer is selected and disabled.

Freestyle relies on render layer name (RenderLayer::name) to find the render layer that
corresponds to a scene render layer being rendered.

When the active render layer is disabled during preview rendering, the function
render_result_new() populates the list of render layers (RenderResult::layers) with an
unnamed render layer while setting the active scene render layer index to zero.

This commit fixes the missing initialization of the render layer name by referring to that of
the first in the list of scene render layers.

source/blender/render/intern/source/render_result.c

@@ -604,6 +604,8 @@ RenderResult *render_result_new(Render *re, rcti *partrct, int crop, int savebuf
 		rl->passflag = SCE_PASS_COMBINED;
 		
 		re->r.actlay = 0;
+		srl = BLI_findlink(&re->r.layers, re->r.actlay);
+		BLI_strncpy(rl->name, srl->name, sizeof(rl->name));
 	}
 	
 	/* border render; calculate offset for use in compositor. compo is centralized coords */