fix spirv compile failure diagnostics read past shader buffer

[lazypool]

Summary

Fixes #6881 — the error-printing path in compile_spirv_module() (src/gpu.cpp) treated the shader source slice as a null-terminated C string when it is not, causing an out-of-bounds read and garbled diagnostic output whenever a Vulkan compute shader fails to compile.

Root cause

• Shader .comp_data[] arrays are generated by cmake/ncnn_generate_shader_comp_header.cmake as hex-encoded bytes without a trailing 0x00.
• compile_spirv_module() slices the source into comp_datas[3] (everything after #version) and, on parse failure, scans it with strchr() / *p != '\0' — both require null-termination, but none is guaranteed.
• In practice strchr() walks past the end of the buffer into adjacent memory, printing garbage (or, with ASan, reporting a stack-buffer-overflow).

Change

Bound the scan using the already-known source length (comp_data_size_2, the size of comp_datas[3]):

• strchr(p, '\n') → memchr(p, '\n', p_end - p)
• final if (*p != '\0') → if (p != p_end)
• final %s print → %.*s with the remaining length

Verification

Built with -DNCNN_VULKAN=ON (system glslang 16.3.0) and tested under AddressSanitizer on RADV:

• Before: compiling a broken, non-null-terminated shader source via compile_spirv_module(data, size, ...) → ERROR: AddressSanitizer: stack-buffer-overflow ... in ncnn::compile_spirv_module(...)
• After: same input prints the shader lines cleanly and bounded (1:\tvoid main() { ... }), no ASan report; a real shader still compiles (rc=0).

No behavioral change to successful compilation paths.

[tencent-adm]

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[lazypool]

Please review when you get a chance, thanks!

[github-actions]

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