fix(region): signed integer overflow in RegionCommonComputeSymbolTimeLoRa at SF11/SF12

[Alexey-Lukin]

Problem

RegionCommonComputeSymbolTimeLoRa() computes:

return ( 1 << phyDr ) * 1000000 / bandwidthInHz;

The multiplication is evaluated in (signed) int arithmetic. For phyDr >= 11 the intermediate product exceeds INT32_MAX (SF11: 2048 * 1e6 = 2.048e9; SF12: 4096 * 1e6 = 4.096e9), which is undefined behavior in C.

On typical two's-complement targets the wrapped intermediate makes the returned symbol time garbage (instead of e.g. 32768 µs for SF12 / 125 kHz), corrupting the RX window timeout/offset computation in RegionCommonComputeRxWindowParameters(). With a different compiler or optimization level the behavior is not defined at all — the optimizer is allowed to assume signed overflow never happens.

Caught by UndefinedBehaviorSanitizer while host-testing the MAC (OTAA join at DR0 / EU868) against a stub radio:

RegionCommon.c: runtime error: signed integer overflow: 4096 * 1000000 cannot be represented in type 'int'
    #0 RegionCommonComputeSymbolTimeLoRa RegionCommon.c
    #1 RegionEU868ComputeRxWindowParameters RegionEU868.c
    #2 ScheduleTx LoRaMac.c
    #3 LoRaMacMlmeRequest LoRaMac.c

Fix

Force the computation into uint32_t:

return ( ( uint32_t )1 << phyDr ) * 1000000U / bandwidthInHz;

The maximum intermediate value (4096 * 1000000 = 4.096e9) fits in an unsigned 32-bit integer, and the function already returns uint32_t.

Affects every region file that reaches this helper with SF11/SF12 (EU868 RX2/DR0 join being the most common path).