Fix crash in sentry_toIso8601String

[philipphofmann]

Description

The SDK sometimes crashes with SIGABRT in sentry_toIso8601String.

Exception Type: EXC_CRASH (SIGABRT)

Thread 0 Crashed:
0   libsystem_kernel.dylib          0x1e54a2974         __pthread_kill
1   libsystem_pthread.dylib         0x1f8ee10e8         pthread_kill
2   libsystem_c.dylib               0x1a4f5bb7c         abort
3   libsystem_malloc.dylib          0x1ad186c64         malloc_vreport
4   libsystem_malloc.dylib          0x1ad186f0c         malloc_zone_error
5   libsystem_malloc.dylib          0x1ad17ca40         nanov2_guard_corruption_detected
6   libsystem_malloc.dylib          0x1ad162f80         nanov2_allocate_outlined
7   CoreFoundation                  0x19cfa8ab4         _CFRuntimeCreateInstance
8   CoreFoundation                  0x19cfa8540         __CFStringCreateImmutableFunnel3
9   CoreFoundation                  0x19d01ef04         CFDateFormatterCreateStringWithAbsoluteTime
10  Foundation                      0x19bedaab4         -[NSDateFormatter stringForObjectValue:]
11  Sentry.framework                0x1078f7670         -[NSDate(SentryExtras) sentry_toIso8601String]
12  Sentry.framework                0x1078e6a58         -[SentryBreadcrumb serialize]
13  Sentry.framework                0x107908c48         -[SentryScope addBreadcrumb:]
14  Sentry.framework                0x1078d9a88         -[SentryHub addBreadcrumb:]
15  Sentry.framework                0x107907bf4         +[SentrySDK addBreadcrumb:]

We also see this in our internal SDK crashes.