make get_length() take a const buffer

lib/ykpiv.c

@@ -61,7 +61,7 @@ static int set_length(unsigned char *buffer, int length) {
   }
 }
 
-static int get_length(unsigned char *buffer, size_t *len) {
+static int get_length(const unsigned char *buffer, size_t *len) {
   if(buffer[0] < 0x81) {
     *len = buffer[0];
     return 1;

tool/util.c

@@ -150,7 +150,7 @@ void dump_hex(const unsigned char *buf, unsigned int len) {
   }
 }
 
-int get_length(unsigned char *buffer, int *len) {
+int get_length(const unsigned char *buffer, int *len) {
   if(buffer[0] < 0x81) {
     *len = buffer[0];
     return 1;

tool/util.h

@@ -51,7 +51,7 @@ typedef union u_APDU APDU;
 
 void dump_hex(unsigned const char*, unsigned int);
 int set_length(unsigned char*, int);
-int get_length(unsigned char*, int*);
+int get_length(const unsigned char*, int*);
 X509_NAME *parse_name(const char*);
 unsigned char get_algorithm(EVP_PKEY*);
 FILE *open_file(const char*, int);