Fix issue with std::map implementation

Author: vincentlaucsb

include/internal/raw_csv_data.cpp

@@ -8,7 +8,7 @@ namespace csv {
     namespace internals {
         CSV_INLINE RawCSVField& CSVFieldList::operator[](size_t n) const {
             const size_t page_no = n / _single_buffer_capacity;
-            const size_t buffer_idx = (page_no < 1) ? n : n % _single_buffer_capacity;
+            const size_t buffer_idx = n % _single_buffer_capacity;
             return this->buffers.at(page_no)[buffer_idx];
         }
 

include/internal/raw_csv_data.hpp

@@ -79,7 +79,12 @@ namespace csv {
 
                 // Recalculate _back pointer to point into OUR buffers, not the moved-from ones
                 if (!this->buffers.empty()) {
-                    _back = this->buffers[_current_block].get() + _current_buffer_size;
+                    auto it = this->buffers.find(_current_block);
+                    if (it != this->buffers.end() && it->second) {
+                        _back = it->second.get() + _current_buffer_size;
+                    } else {
+                        _back = nullptr;
+                    }
                 } else {
                     _back = nullptr;
                 }