[freetype2] master a9793feac: [base] Avoid UB with memcpy

[Werner Lemberg]

branch: master
commit a9793feacefac6d44b761bed12566029f5811063
Author: Ben Wagner <bungeman@chromium.org>
Commit: Ben Wagner <bungeman@chromium.org>

    [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 | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/base/ftstream.c b/src/base/ftstream.c
index 05c563757..64826aceb 100644
--- a/src/base/ftstream.c
+++ b/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;