NetworkPkg/Dhcp6Dxe: Fix sanitizer issues #252

Job	Run time
Analyze (ArmPkg, IA32,X64)	4m 42s
Analyze (CryptoPkg, IA32)	16m 54s
Analyze (CryptoPkg, X64)	18m 2s
Analyze (DynamicTablesPkg, IA32,X64)	11m 37s
Analyze (FatPkg, IA32,X64)	12m 41s
Analyze (FmpDevicePkg, IA32,X64)	10m 51s
Analyze (IntelFsp2Pkg, IA32,X64)	11m 43s
Analyze (IntelFsp2WrapperPkg, IA32,X64)	7m 26s
Analyze (MdeModulePkg, IA32)	14m 43s
Analyze (MdeModulePkg, X64)	15m 34s
Analyze (MdePkg, IA32,X64)	11m 16s
Analyze (PcAtChipsetPkg, IA32,X64)	11m 19s
Analyze (PrmPkg, IA32,X64)	11m 27s
Analyze (SecurityPkg, IA32,X64)	13m 35s
Analyze (ShellPkg, IA32,X64)	14m 5s
Analyze (SourceLevelDebugPkg, IA32,X64)	11m 21s
Analyze (StandaloneMmPkg, IA32,X64)	11m 10s
Analyze (UefiCpuPkg, IA32,X64)	13m 8s
Analyze (UnitTestFrameworkPkg, IA32,X64)	9m 45s
	3h 51m 19s

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