[ATM] Fixed failure when user enters wrong withdraw amount

[maharshijinandra]

When the user enters an invalid withdrawal amount, DispenserAmount() returns an error without releasing the lock, leading to lock contention.
This change fixes the resulting deadlock.

Error output:

ATM state:DispenserAmount
500 note present:1000, 100 not present:1000
 Enter amount to withdraw from atm:
123
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [sync.Mutex.Lock]:
internal/sync.runtime_SemacquireMutex(0x14000078d68?, 0x1c?, 0x102e20268?)
	/opt/homebrew/Cellar/go/1.25.5/libexec/src/runtime/sema.go:95 +0x28
internal/sync.(*Mutex).lockSlow(0x1400010c0e8)
	/opt/homebrew/Cellar/go/1.25.5/libexec/src/internal/sync/mutex.go:149 +0x170
internal/sync.(*Mutex).Lock(...)
	/opt/homebrew/Cellar/go/1.25.5/libexec/src/internal/sync/mutex.go:70
sync.(*Mutex).Lock(...)
	/opt/homebrew/Cellar/go/1.25.5/libexec/src/sync/mutex.go:46
sync.(*RWMutex).Lock(0x1400010c0e8)
	/opt/homebrew/Cellar/go/1.25.5/libexec/src/sync/rwmutex.go:150 +0x6c
atm-machine/atm.(*ATM).CleanPreviousAtmTransaction(0x1400010c000)
	/Users/-/src/low-level-design-golang/atm-machine/atm/atm.go:35 +0x34
atm-machine/atm.(*ATM).Execute(0x1400010c000, 0x1400004e028?)
	/Users/-/src/low-level-design-golang/atm-machine/atm/atm.go:94 +0x3c
main.main()
	/Users/-/src/low-level-design-golang/atm-machine/main.go:46 +0x4a4
exit status 2

cc @thesaltree @codeimmortal