[base] Avoid UB with memcpy

`FT_NEW_ARRAY(p, 0)` sets `p` to `NULL`. `FT_Stream_ReadAt` with a
memory based stream uses `FT_MEM_COPY` which is `memcpy` which specifies
that it is undefined behavior for either the `src` or `dst` to be
`NULL`. Instead of forcing all callers work around calling
`FT_Stream_Read` when `buffer == NULL && count == 0` do the check in
`FT_StreamRead`. This allows any call with `count == 0` to succesfully
read zero bytes without UB.

* src/base/ftstream.c (FT_Stream_ReadAt): skip `FT_MEM_COPY` when
`count == 0`. (FT_Stream_TryRead): ditto

Fixes: #1250

src/base/ftstream.c

@@ -141,7 +141,9 @@
       if ( read_bytes > count )
         read_bytes = count;
 
-      FT_MEM_COPY( buffer, stream->base + pos, read_bytes );
+      /* Allow "reading" zero bytes without UB even if buffer is NULL */
+      if ( count )
+        FT_MEM_COPY( buffer, stream->base + pos, read_bytes );
     }
 
     stream->pos = pos + read_bytes;
@@ -178,7 +180,9 @@
       if ( read_bytes > count )
         read_bytes = count;
 
-      FT_MEM_COPY( buffer, stream->base + stream->pos, read_bytes );
+      /* Allow "reading" zero bytes without UB even if buffer is NULL */
+      if ( count )
+        FT_MEM_COPY( buffer, stream->base + stream->pos, read_bytes );
     }
 
     stream->pos += read_bytes;