Fix potential double-free in test3.c

The pointer to newargv passed to poptParseArgvString() may not be
assigned to in case of an error, and it still may contain an address to
already freed memory from the previous for loop iteration.

To fix, add a return value check, similar to the one above it for the
out pointer.

Found by a static analyzer.

tests/test3.c

@@ -30,7 +30,11 @@ int main (int argc, char **argv) {
 
 	printf ("single string: '%s'\n", out);
 
-	poptParseArgvString (out, &newargc, &newargv);
+	ret = poptParseArgvString (out, &newargc, &newargv);
+	if (ret != 0) {
+	    printf ("cannot parse %s. ret=%d\n", out, ret);
+	    continue;
+	}
 
 	printf ("popt array: size=%d\n", newargc);
 	for (j = 0; j < newargc; j++)