At the end of the realloc function there is call to the free function, the free function calls to the merge function with the parameter "a" (BEFORE(block)) and it happened that "a" points to valid address in non-related memory block. So when the merge function placed some value in a->size, it caused bug.

Actually it is not a bug of DJGPP, but rather my bug, because I overwrite
some memory location that play important role for memory allocation method.
So I withdraw this bug.