Missing ForceZero on plaintext password copy

When a copy of the user's password is freed, it wasn't getting force
zeroed. It might still exist in the heap after getting freed.  Added
a call to `ForceZero()`.

Affected function: CheckPasswordUnix.
Issue: F-56

apps/wolfsshd/auth.c

@@ -430,6 +430,7 @@ static int CheckPasswordUnix(const char* usr, const byte* pw, word32 pwSz, WOLFS
     }
 
     if (pwStr != NULL) {
+        ForceZero(pwStr, pwSz + 1);
         WFREE(pwStr, NULL, DYNTYPE_STRING);
     }
     if (storedHashCpy != NULL) {