| Function silc_buffer_push
 
 SYNOPSIS
 
    static inline
    unsigned char *silc_buffer_push(SilcBuffer sb, SilcUInt32 len);
DESCRIPTION
    Pushes current data area towards beginning. Length of the currently
    valid data area is also incremented. Returns a pointer to the
    data area before pushing. Returns NULL on error.
EXAMPLE
    ---------------------------------
    | head     | data    | tail     |
    ---------------------------------
               ^
               Pushes the start of the data area.
    ---------------------------------
    | head  | data       | tail     |
    ---------------------------------
               ^
    silc_buffer_push(sb, 20);
 
 
 
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