Buffer comparison cast fix

[JensOgorek]

This is a fix for #86.
Size_t is unsigned and pos can be negative. After the change, I could not reproduce the problem.

Also, I increased the size of the patter queue to 100, like in the main queue with uart 0. I ran into a full queue multiple times.
W (3232) uart: Fail to enqueue pattern position, pattern queue is full.

[falkoschindler]

Ok, but there's also

if (num_chars > buffer_len - 1)

in string_utils.cpp and multiple

buffer_len - pos

in expressions.cpp. I wonder if we need casting in these lines as well if mixing int and size_t is an issue. 🤔

[JensOgorek]

The main issue is the comparison of int with size_t, when the int is negative. But it would be good practice not to mix unsigned and signed without casting

[falkoschindler]

I see. Then the comparison addressed in this PR should be the only problematic case (as far as I can see). 👍

[JensOgorek]

Right now, yes. But maybe we should have an eye open for this and change it sometime in the future.