membuffer: use buffer pointer as flexible array member

(cherry picked from commit 9c3669b03f)

An additional change was made to correct an ASAN issue -- the membuffer
is reset following allocation in MemBufferCreateNew().

src/util-buffer.c

@@ -42,14 +42,12 @@ MemBuffer *MemBufferCreateNew(uint32_t size)
 
     size_t total_size = size + sizeof(MemBuffer);
 
-    MemBuffer *buffer = SCMalloc(total_size);
+    MemBuffer *buffer = SCCalloc(1, total_size);
     if (unlikely(buffer == NULL)) {
         sc_errno = SC_ENOMEM;
         return NULL;
     }
     buffer->size = size;
-    buffer->buffer = (uint8_t *)buffer + sizeof(MemBuffer);
-
     return buffer;
 }
 
@@ -75,7 +73,6 @@ int MemBufferExpand(MemBuffer **buffer, uint32_t expand_by) {
     }
     *buffer = tbuffer;
     (*buffer)->size += expand_by;
-    (*buffer)->buffer = (uint8_t *)tbuffer + sizeof(MemBuffer);
 
     SCLogDebug("expanded buffer by %u, size is now %u", expand_by, (*buffer)->size);
     return 0;

src/util-buffer.h

@@ -25,9 +25,9 @@
 #define __UTIL_BUFFER_H__
 
 typedef struct MemBuffer_ {
-   uint8_t *buffer;
-   uint32_t size;
-   uint32_t offset;
+    uint32_t size;
+    uint32_t offset;
+    uint8_t buffer[];
 } MemBuffer;
 
 MemBuffer *MemBufferCreateNew(uint32_t size);