[PATCH] Fix segfault in HashCode() (subtitle: implicit conversions considered harmful)

Hello,

The code below segfaults on Linux when MMGT_OPT is set to 0 or 2:

#include
#include

int main(int argc, char **argv) {
Standard_Transient s;
Standard_Integer i = s.HashCode(1000);
std::cout return 0;
}

The current implementation calls HashCode(Handle_Standard_Transient,Standard_Integer)
due to implicit conversion operator. But if object is allocated on the stack,
a segmentation fault occurs when the Handle_Standard_Transient temporary object
is deleted. This happens if MMGT_OPT is set and different from 1.

Implicit conversions should really be forbidden in order to avoid those subtle bugs.