Unchecked malloc fixes

Miscellaneous unchecked malloc fixes. Also fixed some mem leaks on error
paths as I spotted them along the way.

Reviewed-by: Tim Hudson <tjh@openssl.org>

apps/s_client.c

@@ -550,6 +550,11 @@ static char *ssl_give_srp_client_pwd_cb(SSL *s, void *arg)
     PW_CB_DATA cb_tmp;
     int l;
 
+    if(!pass) {
+        BIO_printf(bio_err, "Malloc failure\n");
+        return NULL;
+    }
+
     cb_tmp.password = (char *)srp_arg->srppassin;
     cb_tmp.prompt_info = "SRP user";
     if ((l = password_callback(pass, PWD_STRLEN, 0, &cb_tmp)) < 0) {