fix: Wire::writeReadAsync always malloc'ing DMA buffer

[veecle-stefan]

The async implementation of I2C (Wire) seems to allocate the DMA buffer for every call because the check, whether the current buffer is large enough (if (_dmaSendBufferLen < bufLen) {) is always true because _dmaSendBufferLen is never actually set anywhere.

Simple fix: added a line after successful malloc that sets the _dmaSendBufferLen = bufLen.

PS: Shouldn't we use new uint16_t[bufLen] here, since we're in a C++ object? It probably maps to malloc but it would at least avoid one cast.

[earlephilhower]

Great catch, thanks!

I'm fine leaving malloc/free in place of new[]/delete[] here. For primitives there's no consistency here and it depends on what I was feeling like the day I wrote the code. The one gotcha, though, is that new could throw which we can't catch (unless you explicitly enable exceptions in the IDE and blow up your code size). They map down to malloc and free in any case, like you said...