Correcting an UBSan error in builder

[bjosv]

If a table with optional fields is created before the ds stack is allocated we get an UBSan error.
The ds stack is normally allocated when the first field is added to the table.

Reproduced by adding some steps in test optional_scalars_test.
See #343 for additional fixes.

/xxxx/flatcc/src/runtime/builder.c:613:16: runtime error: null pointer passed as argument 1, which is declared to never be null /usr/include/string.h:61:62: note: nonnull attribute specified here
    #0 0x600c6b6e in exit_frame /xxxx/flatcc/src/runtime/builder.c:613:9
    #1 0x600d4520 in flatcc_builder_end_table /xxxx/flatcc/src/runtime/builder.c:1380:5
    #2 0x600a0be8 in optional_scalars_NestedTable_end /xxxx/flatcc/build/Debug/test/optional_scalars_test/generated/optional_scalars_test_builder.h:35:1
    #3 0x600a08a7 in create_scalar_stuff /xxxx/flatcc/test/optional_scalars_test/optional_scalars_test.c:27:20
    #4 0x600ab4f5 in test /xxxx/flatcc/test/optional_scalars_test/optional_scalars_test.c:176:5
    #5 0x600abd1f in main /xxxx/flatcc/test/optional_scalars_test/optional_scalars_test.c:272:9
    #6 0xe9186518  (/lib/i386-linux-gnu/libc.so.6+0x21518) (BuildId: 6f8a2d2f90a25e7865aa6fdfd9a7825d62d53f51)
    #7 0xe91865f2 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x215f2) (BuildId: 6f8a2d2f90a25e7865aa6fdfd9a7825d62d53f51)
    #8 0x6007053a in _start (/xxxx/flatcc/build/Debug/test/optional_scalars_test/optional_scalars_test_d+0x2653a) (BuildId: d47f8a02ee936bc1bc53a0efd6ad360faa6428d7)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /xxxx/flatcc/src/runtime/builder.c:613:16

This probably need an extra pair of eyes from an expert..