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  xalloc - safe memory allocation
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this module implements malloc and friends + some extensions.
if __DEBUG__ symbol is defined. the functions will provide extended
debug logic and kills the program (passing 0 as size to malloc for example).

the funtions will at all times kill the program if memory can't be allocated for
some reason, this makes the need for client-code to check for null pointers returned
by these functions redundant.

-- functions

xmalloc():

    just like malloc, this function allocates a block of memory of 'size' bytes.
    if compiled with the __DEBUG__ symbol, the function will not allow zero size

xmallocz():

    exactly like xmalloc but will initialize the block with zero's.

xrealloc():

    reallocates a previous allocated block of memory to 'size' bytes.
    if compiled with the __DEBUG__ symbol, the function will not allow zero size

xstrdup():

    allocates and copies the string 's' to a new memory location and returns it to the user.
    if compiled with the __DEBUG__ symbol, the function will not allow 's' to be a NULL pointer

xfree():

    free's a previous allocated block (pointed to by 'ptr') that is allocated by xmalloc/malloc.
    if compiled with the __DEBUG__ symbol, the function will not allow 'ptr' to be a NULL pointer