fix: handle reallocation failure gracefully in DataPointer::resize()

Immediate reassignment leaves the `buf.data` pointer with a nullptr,
leaking the original data. Furthermore, it leaves the length field
inconsistent.
This patch first checks whether the pointer was a nullptr before
reassigning.

Author: ndossche

include/ncrypto.h

@@ -690,6 +690,8 @@ class DataPointer final {
   bool isSecure() const { return secure_; }
 
  private:
+  void free();
+
   void* data_ = nullptr;
   size_t len_ = 0;
   bool secure_ = false;

src/ncrypto.cpp

@@ -228,14 +228,18 @@ void DataPointer::zero() {
   OPENSSL_cleanse(data_, len_);
 }
 
-void DataPointer::reset(void* data, size_t length) {
+void DataPointer::free() {
   if (data_ != nullptr) {
     if (secure_) {
       OPENSSL_secure_clear_free(data_, len_);
     } else {
       OPENSSL_clear_free(data_, len_);
     }
   }
+}
+
+void DataPointer::reset(void* data, size_t length) {
+  free();
   data_ = data;
   len_ = length;
 }
@@ -258,7 +262,12 @@ DataPointer DataPointer::resize(size_t len) {
   size_t actual_len = std::min(len_, len);
   auto buf = release();
   if (actual_len == len_) return DataPointer(buf.data, actual_len);
-  buf.data = OPENSSL_realloc(buf.data, actual_len);
+  auto new_data = OPENSSL_realloc(buf.data, actual_len);
+  if (new_data == nullptr) {
+    free();
+    return {};
+  }
+  buf.data = new_data;
   buf.len = actual_len;
   return DataPointer(buf);
 }