hush: Fix assignments being misinterpreted as commands

If there were no variable substitutions in a command, then initial
assignments would be misinterpreted as commands, instead of being skipped
over. This is demonstrated by the following example:

	=> foo=bar echo baz
	Unknown command 'foo=bar' - try 'help'

Signed-off-by: Sean Anderson <seanga2@gmail.com>

test/cmd/test_echo.c

@@ -34,6 +34,8 @@ static struct test_data echo_data[] = {
 	 */
 	{"setenv jQx X; echo \"a)\" ${jQx} 'b)' '${jQx}' c) ${jQx}; setenv jQx",
 	 "a) X b) ${jQx} c) X"},
+	/* Test shell variable assignments without substitutions */
+	{"foo=bar echo baz", "baz"},
 	/* Test handling of shell variables. */
 	{"setenv jQx; for jQx in 1 2 3; do echo -n \"${jQx}, \"; done; echo;",
 	 "1, 2, 3, "},