When g_num_exts=0, the input of xwr_softpll_ng
clk_ext_mul_i        : in std_logic_vector(g_num_exts-1 downto 0);
was of wrong range, i.e. std_logic_vector(-1 to 0).

Function f_nonzero_vector() was added to generate
std_logic_vector(0 to 0) in the case when g_num_exts=0.