Fixes for some more exotic warnings

src/civetweb.c

@@ -12398,6 +12398,7 @@ sslize(struct mg_connection *conn,
 				/* This is an IO error. Look at errno. */
 				err = errno;
 				/* TODO: set some error message */
+				(void)err;
 				break;
 			} else {
 				/* This is an SSL specific error */
@@ -15379,6 +15380,13 @@ mg_get_system_info_impl(char *buffer, int buflen)
 	const char *eol = "\n";
 #endif
 
+	if (buffer == NULL) {
+		/* Avoid some warning (although, if some dillweed supplies
+		 * buffer==NULL combined with buflen>0, he deserves a crash).
+		 */
+		buflen = 0;
+	}
+
 	/* Server version */
 	{
 		const char *version = mg_version();

src/handle_form.inl

@@ -650,6 +650,8 @@ mg_handle_form_request(struct mg_connection *conn,
 			/* This line is not required, but otherwise some compilers
 			 * generate spurious warnings. */
 			nend = nbeg;
+			/* And others complain, the result is unused. */
+			(void)nend;
 
 			/* If name=" is found, search for the closing " */
 			if (nbeg) {