mi: fix memory leak in miFillUniqueSpanGroup

This is based off the corresponding xserver commit from Tiago Vignatti:
http://cgit.freedesktop.org/xorg/xserver/commit/mi/mispans.c?id=7ae46b69ba3f05f46529131e6a864904967cde3a

Since xrealloc is #defined to spice_realloc which aborts on failure, this
block is dead code, but I prefer to stay as close as possible to the
upstream xserver code this is based on.

common/lines.c

@@ -814,6 +814,8 @@ miFillUniqueSpanGroup (GCPtr pGC, SpanGroup * spanGroup, Boolean foreground)
                             }
                             xfree (yspans);
                             xfree (ysizes);
+                            xfree (newpoints);
+                            xfree (newwidths);
                             miDisposeSpanGroup (spanGroup);
                             return;
                         }