Fix Windows regression

commit cb9f919f ("gdb: add program_space parameter to
lookup_minimal_symbol_text") caused a crash on Windows.  In this
function, section can be nullptr, but it is unconditionally
dereferenced by the change introduced by the patch.

I tested this using the AdaCore internal test suite.

v2: always use current_program_space, reverting to be behavior before
cb9f919f.

Approved-By: Simon Marchi <simon.marchi@efficios.com>