[C++] Fix null pointer dereference in SimpleTable constructor

The SimpleTable constructors dereference columns_[0] and columns[0]
without checking for null when num_rows < 0. This causes a segfault
if a user passes a vector containing null pointers, as the null check
in ValidateMeta() runs after construction.

Fixed by adding null checks before dereferencing the first column.

Author: Alirana2829

cpp/src/arrow/table.cc

@@ -63,7 +63,7 @@ class SimpleTable : public Table {
       : columns_(std::move(columns)) {
     schema_ = std::move(schema);
     if (num_rows < 0) {
-      if (columns_.size() == 0) {
+      if (columns_.size() == 0 || !columns_[0]) {
         num_rows_ = 0;
       } else {
         num_rows_ = columns_[0]->length();
@@ -77,7 +77,7 @@ class SimpleTable : public Table {
               const std::vector<std::shared_ptr<Array>>& columns, int64_t num_rows = -1) {
     schema_ = std::move(schema);
     if (num_rows < 0) {
-      if (columns.size() == 0) {
+      if (columns.size() == 0 || !columns[0]) {
         num_rows_ = 0;
       } else {
         num_rows_ = columns[0]->length();