Fix memory leaks in vipsGetPoint and vipsImageGetAsString

[davidbyttow]

Summary

• vipsGetPoint: The deferred g_free captured a nil pointer before the C call populated it, so the C-allocated array was never freed. The returned Go slice was also backed by C memory. Fixed by copying into a Go-owned slice and freeing immediately.
• vipsImageGetAsString: Same defer-captures-nil pattern. The g_free-allocated string from vips_image_get_as_string was never freed. Fixed by wrapping the defer in a closure.

Both bugs stem from Go's defer f(x) evaluating x at the defer statement, not at function exit.

Test plan

• TestGetPoint_RepeatedCalls - calls GetPoint 10 times, retains all results, verifies correctness
• TestGetAsString_RoundTrip - sets a string field, reads it back with GetAsString

🤖 Generated with Claude Code

Note

Fix memory leaks by returning Go-owned slices from vips.GetPoint and freeing C strings in vips.ImageGetAsString

Adjust vips.GetPoint to copy C buffers into Go slices and free the C memory, and update vips.ImageGetAsString to defer freeing the C string with the final pointer value. Add tests for repeated GetPoint(10,10) calls and string metadata round-trip in image_test.go. Modify implementations in operations.go.

📍Where to Start

Start with vipsGetPoint and vipsImageGetAsString in operations.go.

^{Macroscope summarized 381b362.}

[coveralls]

coverage: 36.732% (+0.4%) from 36.365%
when pulling 381b362 on fix/memory-leaks
into 12bbea8 on master.