crypto: fix assert() on malformed hex input

Use the StringBytes::IsValidString() function introduced in commit
dce26cce to ensure that the input string meets the expectations of the
other StringBytes functions before processing it further.

Fixes the following assertion:

    Assertion failed: (str->Length() % 2 == 0 && "invalid hex string
    length"), function StorageSize, file ../../src/string_bytes.cc,
    line 301.

Fixes #5725.