[Bug] Fix mem_pool npe (#4045)

Fix mem_pool NPE in column reader.
Add a safe allocation method.

be/src/olap/rowset/column_reader.cpp

@@ -206,7 +206,13 @@ OLAPStatus StringColumnDirectReader::next_vector(
             string_buffer_size += length;
         }
 
-        char* string_buffer = reinterpret_cast<char*>(mem_pool->allocate(string_buffer_size));
+        uint8_t* allocated_mem;
+        res = mem_pool->allocate_safely(string_buffer_size, allocated_mem);
+        if (OLAP_SUCCESS != res) {
+            return res;
+        }
+        char* string_buffer = reinterpret_cast<char*>(allocated_mem);
+
         for (int i = 0; i < size; ++i) {
             length = _values[i].size;
             if (UNLIKELY(length == 0)) {
@@ -239,7 +245,13 @@ OLAPStatus StringColumnDirectReader::next_vector(
             }
         }
 
-        char* string_buffer = reinterpret_cast<char*>(mem_pool->allocate(string_buffer_size));
+        uint8_t* allocated_mem;
+        res = mem_pool->allocate_safely(string_buffer_size, allocated_mem);
+        if (OLAP_SUCCESS != res) {
+            return res;
+        }
+        char* string_buffer = reinterpret_cast<char*>(allocated_mem);
+
         for (int i = 0; i < size; ++i) {
             if (!is_null[i]) {
                 length = _values[i].size;

be/src/runtime/mem_pool.h

@@ -30,6 +30,7 @@
 #include "gutil/dynamic_annotations.h"
 #include "util/bit_util.h"
 #include "runtime/memory/chunk.h"
+#include "olap/olap_define.h"
 
 namespace doris {
 
@@ -110,6 +111,11 @@ public:
         return allocate<false>(size, DEFAULT_ALIGNMENT);
     }
 
+    /// Same as Allocate() excpect add a check when return a nullptr
+    OLAPStatus allocate_safely(int64_t size, uint8_t*& ret) {
+        return allocate_safely<false>(size, DEFAULT_ALIGNMENT, ret);
+    }
+
     /// Same as Allocate() except the mem limit is checked before the allocation and
     /// this call will fail (returns NULL) if it does.
     /// The caller must handle the NULL case. This should be used for allocations
@@ -242,6 +248,16 @@ private:
         return result;
     }
 
+    template <bool CHECK_LIMIT_FIRST>
+    OLAPStatus ALWAYS_INLINE allocate_safely(int64_t size, int alignment, uint8_t*& ret) {
+        uint8_t* result = allocate<CHECK_LIMIT_FIRST>(size, alignment);
+        if (result == nullptr) {
+            return OLAP_ERR_MALLOC_ERROR;
+        }
+        ret = result;
+        return OLAP_SUCCESS;
+    }
+
 private:
     /// chunk from which we served the last Allocate() call;
     /// always points to the last chunk that contains allocated data;