Experiment: limit size of strings read from lib/symbols

Experiment: limit size of strings read from lib/symbols

Coverity complains that we might read strings that are arbitrary
large, and that this can be a security issue. This is a problem in
particular, when we feed these strings to from_utf8(), which coverity
flags as dangerous for some reason.

The best solution would be IMO to model from_utf8() properly, but I do
not know how to do that. Here I try a different solution, where I
cannot read a string larger than 64k from the file.

Let's see whether this removes part of coverity warnings.