fix: check key type in externals dict

[vthib]

The type of the key for the externals dict in both the compile and the match function was not checked. Providing a dict with non string keys leads to a segfault:

For example:

import yara

rules = yara.compile(source="", externals={ "a": 2 })
rules.match(data="", externals={ 1: 2 })