Fix crashes when vkAllocateMemory fails

[Upliner]

When system doesn't have enough VRAM, vkAllocateMemory fails and my app crashes. This patch fixes this by always checking return value of allocate_memory and allocate_dedicated_memory so my app can properly report error instead of crashing.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 15.62500% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.28%. Comparing base (419db5e) to head (2212de3).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/allocator.cpp	15.62%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5754      +/-   ##
==========================================
- Coverage   93.29%   93.28%   -0.01%     
==========================================
  Files         845      845              
  Lines      265560   265757     +197     
==========================================
+ Hits       247743   247905     +162     
- Misses      17817    17852      +35     

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[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ Upliner
❌ nihui
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15155464	15155464	0 😘
armhf	6182208	6182336	+128 ⚠️
aarch64	9520608	9454976	-65632 😘

[Copilot]

Pull request overview

This pull request fixes crashes that occur when Vulkan memory allocation fails due to insufficient VRAM by adding proper error handling and cleanup.

• Adds null checks after all allocate_memory and allocate_dedicated_memory calls
• Ensures proper resource cleanup (Vulkan object destruction and memory deallocation) on allocation failure
• Prevents crashes by returning null instead of proceeding with invalid memory

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[nihui]

Thanks for your contribution !