Fix uninitialized memory in C API (#98)

`AllocHGlobal` does not zero-initialize the memory it allocates. This
change fixes the `RecordBatch` exporter to correctly set the "nulls"
buffer to be null instead of inheriting what it got from allocation.

Author: CurtHagenlocher

src/Apache.Arrow/C/CArrowArrayExporter.cs

@@ -184,6 +184,7 @@ private unsafe static void ConvertRecordBatch(ExportedAllocationOwner sharedOwne
 
             cArray->n_buffers = 1;
             cArray->buffers = (byte**)sharedOwner.Allocate(IntPtr.Size);
+            cArray->buffers[0] = null;
 
             cArray->n_children = batch.ColumnCount;
             cArray->children = null;