util: introduce xstrndup helper

We already have xstrdup, add xstrndup as well to make it
straight-forward to clone part of a string.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

util.c

@@ -33,6 +33,17 @@ char *xstrdup(const char *s)
 	return d;
 }
 
+char *xstrndup(const char *s, size_t n)
+{
+	size_t len = strnlen(s, n) + 1;
+	char *d = xmalloc(len);
+
+	memcpy(d, s, len - 1);
+	d[len - 1] = '\0';
+
+	return d;
+}
+
 int xavsprintf_append(char **strp, const char *fmt, va_list ap)
 {
 	int n, size = 0;	/* start with 128 bytes */

util.h

@@ -61,6 +61,7 @@ static inline void *xrealloc(void *p, size_t len)
 }
 
 extern char *xstrdup(const char *s);
+extern char *xstrndup(const char *s, size_t len);
 
 extern int PRINTF(2, 3) xasprintf(char **strp, const char *fmt, ...);
 extern int PRINTF(2, 3) xasprintf_append(char **strp, const char *fmt, ...);