| Function silc_buffer_push_tail
 
 SYNOPSIS
 
    static inline
    unsigned char *silc_buffer_push_tail(SilcBuffer sb, SilcUInt32 len);
DESCRIPTION
    Pushes current tail section towards beginning. Length of the current
    valid data area is also decremented. Returns a pointer to the
    tail section before pushing. Returns NULL on error.
EXAMPLE
    ---------------------------------
    | head  | data           | tail |
    ---------------------------------
                             ^
                             Pushes the start of the tail section.
    ---------------------------------
    | head  | data       | tail     |
    ---------------------------------
                             ^
    silc_buffer_push_tail(sb, 23);
 
 
 
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