crypto: fix signed/unsigned comparison warning

The type of the expression `(uint16_t) server_names_len + 2` gets
implicitly widened to int. Change the type of server_names_len to
uint32_t to avoid the following warnings:

    ../../src/node_crypto_clienthello.cc:144: warning: comparison
    between signed and unsigned integer expressions
    ../../src/node_crypto_clienthello.cc:146: warning: comparison
    between signed and unsigned integer expressions