[PXEXT] Avoid returning a false EOF when we have an empty buffer queued

src/core/posix_io.c

 			free_iob ( iobuf );
 		}
 		file->pos += len;
-		return len;
+		if ( len )
+			return len;
+		break;
 	}
 
 	/* If file has completed, return (after returning all data) */