shell: print zeroes when the buffer size is not a multiple of 16

shell.c

@@ -62,13 +62,14 @@ static void print_hex_ascii(FILE *fp, const char *buf, size_t len)
 
 	for (j = 0; j < len; j += 16, buf += 16) {
 		for (i = 0; i < 16; ++i) {
-			fprintf(fp, "%02x", buf[i]);
+			c = (i < len) ? buf[i] : 0;
+			fprintf(fp, "%02x", c);
 		}
 
 		fprintf(fp, " ");
 
 		for (i = 0; i < 16; ++i) {
-			c = buf[i];
+			c = (i < len) ? buf[i] : 0;
 			fprintf(fp, "%c", isalnum(c) ? c : '.');
 		}