fdo#37326: Sign number must be either 1 or -1, never 0.

This fixes incorrect result with LINEST cell function.
Signed-off-by: Kohei Yoshida <kyoshida@novell.com>

sc/source/core/tool/interpr5.cxx

@@ -1978,14 +1978,10 @@ double lcl_TGetColumnSumProduct(ScMatrixRef pMatA, SCSIZE nRa,
     return fResult;
 }
 
+// no mathematical signum, but used to switch between adding and subtracting
 double lcl_GetSign(double fValue)
 {
-    if (fValue < 0.0)
-        return -1.0;
-    else if (fValue > 0.0)
-        return 1.0;
-    else
-        return 0.0;
+    return (fValue >= 0.0 ? 1.0 : -1.0 );
 }
 
 /* Calculates a QR decomposition with Householder reflection.