tal/str: fix error in tal_strndup()

It used strlen() on the source, which might not be valid.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/str/str.c

@@ -26,11 +26,9 @@ char *tal_strndup(const tal_t *ctx, const char *p, size_t n)
 	char *ret;
 
 	/* We have to let through NULL for take(). */
-	if (likely(p)) {
-		len = strlen(p);
-		if (len > n)
-			len = n;
-	} else
+	if (likely(p))
+		len = strnlen(p, n);
+	else
 		len = n;
 
 	ret = tal_dup_(ctx, p, 1, len, 1, false, TAL_LABEL(char, "[]"));

ccan/tal/str/test/run-strndup.c

+#include <ccan/tal/str/str.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <ccan/tal/str/str.c>
+#include <ccan/tap/tap.h>
+#include "helper.h"
+
+int main(int argc, char *argv[])
+{
+	char *str, *copy;
+
+	plan_tests(1);
+	str = malloc(5);
+	memcpy(str, "hello", 5);
+	/* We should be fine to strndup src without nul terminator. */
+	copy = tal_strndup(NULL, str, 5);
+	ok1(!strcmp(copy, "hello"));
+	tal_free(copy);
+	free(str);
+
+	return exit_status();
+}