Add advanced calloc and realloc features (#41)

## Description

Add advanced calloc and realloc features.

## Issues

<!-- use if this PR fully resolves the issue -->

Closes #33 

<!-- use if this PR is linked but should not close the issue -->

Related: #<issue-number>

## Testing

<!-- Steps or notes on how reviewers can verify. -->

Author: felixnagele

src/calloc.cpp

@@ -1,14 +1,20 @@
 #include "malloc_from_scratch/memory_allocator.h"
+
 #include <cstring>
 
 namespace mem
 {
 
 void* calloc(size_t num, size_t size)
 {
+    if (num == 0 || size == 0)
+    {
+        return nullptr;
+    }
+
     size_t total_size = num * size;
     size_t overflow_check = total_size / num;
-    if (overflow_check != size && num > 0 && size > 0)
+    if (overflow_check != size)
     {
         return nullptr;
     }

src/realloc.cpp

@@ -1,4 +1,8 @@
 #include "malloc_from_scratch/memory_allocator.h"
+#include "malloc_from_scratch/memory_internal.h"
+
+#include <cstdlib>
+#include <cstring>
 
 namespace mem
 {
@@ -16,12 +20,41 @@ void* realloc(void* ptr, size_t new_size)
         return nullptr;
     }
 
+    if (!internal::isPointerInHeap(ptr))
+    {
+        return nullptr;
+    }
+
+    internal::MemoryBlock* old_block = internal::getMetadata(ptr);
+    if (!internal::isValidBlock(old_block))
+    {
+        return nullptr;
+    }
+
+    if (!internal::checkCanary(old_block))
+    {
+        exit(-1);
+    }
+
+    size_t old_size = old_block->size_;
+
     void* new_ptr = malloc(new_size);
     if (new_ptr == nullptr)
     {
         return nullptr;
     }
 
+    size_t copy_size;
+    if (old_size < new_size)
+    {
+        copy_size = old_size;
+    }
+    else
+    {
+        copy_size = new_size;
+    }
+    memcpy(new_ptr, ptr, copy_size);
+
     free(ptr);
 
     return new_ptr;