ord: fix a crash when link_design is called twice.

[mikesinouye]

Summary

OpenROAD throws sigsegv when link_design is called twice due to the sta::VerilogReader being destructed but the openroad object retains the dangling pointer.

Repro:

read_lef test/sky130hd/sky130hd.tlef
read_lef test/sky130hd/sky130_fd_sc_hd.lef
read_liberty test/sky130hd/sky130hd_tt.lib
read_verilog test/gcd_sky130hd.v
link_design gcd

# allocate over the existing memory
report_wns

# crashes
link_design gcd

Type of Change

• Bug fix

Impact

STA will throw an error instead of crashing.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have signed my commits (DCO).

[gemini-code-assist]

Code Review

This pull request updates the linkDesign method in src/OpenRoad.cc to ensure that the link function in verilog_network_ is reset to nullptr after a successful design link. I have no feedback to provide as there were no review comments.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"