mem: new flb_realloc_z() function
mem: new flb_realloc_z() function

the following patch introduce a new wrapper for memory reallocation that
in addition to realloc a memory buffer, if the size is extended, it will
reset the new extended memory region with zeros.

Signed-off-by: Eduardo Silva <81f705dc2ce1a61a2621e0e4b442a9474e1d0c70@treasure-data.com>

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