gdb: revert one erroneous bool-ification change

Commit 42c13555ff88 ("Change value::m_stack to bool") erroneously
changed a `0` to `false` in this call to read_value_memory.  This
parameter is `LONGEST bit_offset`, it should stay `0`.

Change-Id: I128df6834cf8055ec6a7051e237e379978d3d651
1 file changed