Prevent a few edge case overflows

src/arena.c

@@ -60,10 +60,10 @@ void *arena_push(Arena *arena, U32 size, U32 align)
     Assert(IsPow2(align));
 
     MemoryBlock *block = arena->current;
-    U32 start_pos = AlignUpPow2(block->pos, align);
+    U32 start_pos = AlignUpPow2(block->pos, align);  // might align past block->size
     U32 available = block->size - start_pos;
 
-    if (available < size) {
+    if (start_pos > block->size || available < size) {
         U32 block_size = arena->block_size; // original user requested size
         U32 new_commit_size = arena->commit_size;
         if (size > block_size - MemoryBlock_Header_Size) {

src/arena.h

@@ -16,7 +16,7 @@ struct MemoryBlock {
     U8 *base;
     U32 size;           // Block size, page aligned
     U32 pos;            // start of next allocation
-    U32 base_pos;       // absolute position of base relative to the first linked arena
+    U64 base_pos;       // absolute position of base relative to the first linked arena
     U32 committed_pos;  // end of committed region. Will be page aligned
     MemoryBlock *prev;
 };

src/main.c

@@ -13,7 +13,7 @@ void arena_dump(const Arena *arena)
         printf("base:        %p\n", block->base);
         printf("size:        %u\n", block->size);
         printf("pos:         %u\n", block->pos);
-        printf("base_pos:    %u\n", block->base_pos);
+        printf("base_pos:    %lu\n",block->base_pos);
         printf("prev: %p\n", block->prev);
         puts("-----Arena  end-");
         block = block->prev;