libfdt: Fix segfault when calling fit_check_format() on corrupt FIT images

It has been observed that fit_check_format() will fail when passed a
corrupt FIT image.  This was tracked down to _fdt_string_eq():
return (strlen(p) == len) && (memcmp(p, s, len) == 0);

In the case of a corrupt FIT image one can't depend on 'p' being NULL
terminated.  I changed it to use strnlen() to fix the issue.

Signed-off-by: Tom Rini <trini@ti.com>

lib/libfdt/fdt_ro.c

@@ -44,7 +44,7 @@ static int _fdt_string_eq(const void *fdt, int stroffset,
 {
 	const char *p = fdt_string(fdt, stroffset);
 
-	return (strlen(p) == len) && (memcmp(p, s, len) == 0);
+	return (strnlen(p, len + 1) == len) && (memcmp(p, s, len) == 0);
 }
 
 int fdt_get_mem_rsv(const void *fdt, int n, uint64_t *address, uint64_t *size)