fix: clean up DIB allocation failures

[sit-d]

Summary

• clear the stale selected-bitmap handle after replacing an existing DIB cache
• release a partially-created DIB section if CreateDIBSection or CreateCompatibleDC fails
• reset cache dimensions on failure so later paints do not see stale buffer metadata

Verification

• Ad-hoc source check confirmed the dib_resize failure paths release partial allocations and reset cache state.
• git diff --check HEAD -- iterator_gdi.c passed.

Notes

• Full Windows GDI build/runtime validation was not run in this Linux environment.