log: Null terminate the errors from OpenSSL

Thanks oss-fuzz

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67807

src/libopensc/log.c

@@ -75,7 +75,7 @@ void sc_do_log_color(sc_context_t *ctx, int level, const char *file, int line, c
 void sc_do_log_openssl(sc_context_t *ctx, int level, const char *file, int line, const char *func)
 {
  BIO *bio = NULL;
- int length = 0;
+ int length, rc;
  char *buffer = NULL;
 
  if ((bio = BIO_new(BIO_s_mem())) == NULL) {
@@ -84,10 +84,21 @@ void sc_do_log_openssl(sc_context_t *ctx, int level, const char *file, int line,
  }
  ERR_print_errors(bio);
 
- if ((length = BIO_pending(bio)) <= 0 ||
- (buffer = malloc(length)) == NULL ||
- BIO_read(bio, buffer, length) <= 0) {
- sc_do_log(ctx, level, file, line, func, "Cannot log OpenSSL error");
+ length = BIO_pending(bio);
+ if (length <= 0) {
+ /* no error? */
+ goto end;
+ }
+ /* trailing null byte */
+ buffer = malloc(length + 1);
+ if (buffer == NULL) {
+ sc_do_log(ctx, level, file, line, func, "No memory!");
+ goto end;
+ }
+ rc = BIO_read(bio, buffer, length);
+ buffer[length] = '\0';
+ if (rc <= 0) {
+ sc_do_log(ctx, level, file, line, func, "Cannot read OpenSSL error");
  goto end;
  }