CLEANUP: arg: remove extra check in make_arg_list arg escaping

Len cannot be equal to 1 when entering in escape handling code.
But yet, an extra "len == 1" check was performed.

Removing this useless check.

This was reported by Ilya with the help of cppcheck.
diff --git a/src/arg.c b/src/arg.c
index 17f5ed0..2810050 100644
--- a/src/arg.c
+++ b/src/arg.c
@@ -182,7 +182,7 @@
 			}
 			else if (*in == '\\' && !squote && len != 1) {
 				/* '\', ', ' ', '"' support being escaped by '\' */
-				if (len == 1 || in[1] == 0)
+				if (in[1] == 0)
 					goto unquote_err;
 
 				if (in[1] == '\\' || in[1] == ' ' || in[1] == '"' || in[1] == '\'') {