NormAngle180: avoid two extra operations

Change-Id: Ib661c57c652c407baef3b6b7390be731fa83ab5d
Reviewed-on: https://gerrit.libreoffice.org/58545
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

svx/source/svdraw/svdtrans.cxx

@@ -396,7 +396,7 @@ long GetAngle(const Point& rPnt)
 
 long NormAngle180(long a)
 {
-    while (a<18000) a+=36000;
+    while (a<-18000) a+=36000;
     while (a>=18000) a-=36000;
     return a;
 }