Fix unnecessary buffer reallocation in sculpt mode, causing an assert

Probably did not cause an actual, the assert is a performance warning.

Ref T76858

source/blender/gpu/intern/gpu_buffers.c

@@ -151,7 +151,10 @@ static bool gpu_pbvh_vert_buf_data_set(GPU_PBVH_Buffers *buffers, uint vert_len)
     /* Initialize vertex buffer (match 'VertexBufferFormat'). */
     buffers->vert_buf = GPU_vertbuf_create_with_format_ex(&g_vbo_id.format, GPU_USAGE_STATIC);
   }
-  GPU_vertbuf_data_alloc(buffers->vert_buf, vert_len);
+  if (buffers->vert_buf->data == NULL || buffers->vert_buf->vertex_len != vert_len) {
+    /* Allocate buffer if not allocated yet or size changed. */
+    GPU_vertbuf_data_alloc(buffers->vert_buf, vert_len);
+  }
 #endif
 
   return buffers->vert_buf->data != NULL;