Fix return value of genRHS python wrapper

Test

wrappers/python/spm/spm.py

@@ -264,7 +264,10 @@ class spmatrix():
 
         info = pyspm_spmGenRHS( rhstype, nrhs, self.id_ptr,
                                 xptr, ldx, b.ctypes.data_as( c_void_p ), ldb )
-        return x, b
+        if getx:
+            return x, b
+        else:
+            return b
 
     def mult( self, B, C, trans=trans.NoTrans, n=-1, alpha=1.0, beta=0. ):