NetworkPkg/IScsiDxe: Remove redundant call to StrLen
NetworkPkg/IScsiDxe: Remove redundant call to StrLen

The commits utilizes a local variable to store the length of a string
which will be used right after.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <f387b71200577e27ccf5abb090fda0ddd187ec6e@intel.com>
Reviewed-by: Fu Siyuan <eef549df2036636f79f66102b4e2dc444721df8f@intel.com>
Reviewed-by: Wu Jiaxin <91f509c31e98a6df13950454fe51aea543223eaa@intel.com>

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