fix typo in Shear negation operator that prevented compilation

(#134)

include/galsim/Shear.h

@@ -111,7 +111,7 @@ namespace galsim {
         {
             double esq = getESq();
             double scale = (esq>1.e-6) ? (1.-std::sqrt(1.-esq))/esq : 0.5;
-            return e>0. ? Shear(-e1*scale, -e2*scale) : Shear(0.0, 0.0);
+            return esq>0. ? Shear(-e1*scale, -e2*scale) : Shear(0.0, 0.0);
         }
 
         // Composition operation: returns ellipticity of