Fix crash in SentryTracer cancelDeadlineTimer

[philipphofmann]

Description

The SDK sometimes crashes in SentryTracer cancelDeadlineTimer with EXC_BAD_ACCESS. Sample stack trace from our internal SDK crash detection:

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: SEGV_NOOP

Thread 0 Crashed:
0   libobjc.A.dylib                 0x18552bc20         objc_msgSend
1   Sentry.framework                0x10625e888         -[SentryTracer cancelDeadlineTimer] (Sentry.framework:249)
2   Sentry.framework                0x10625fa54         -[SentryTracer finishInternal] (Sentry.framework:427)
3   Sentry.framework                0x10625e280         __35-[SentryTracer dispatchIdleTimeout]_block_invoke (Sentry.framework:179)

Could be related to #3977.

[kahest]

@philipphofmann is this a duplicate of #4510?