httpd: brace the empty else body so the host tests build clean

gcc flags the else whose only statement is dbg_string(), because that macro
expands to nothing unless DEBUG is on, leaving the branch with an empty body.
The line is older than this branch; the bench is simply the first thing that
compiles httpd.c with a host compiler and -Wall -Wextra.

Turn warnings into errors in the test Makefile too, so the next one is caught
where it appears instead of scrolling past. It is the only warning the four
test binaries produce, so nothing else has to change.

httpd/httpd.c

@@ -348,8 +348,9 @@ __xdata uint8_t *scan_header(__xdata uint8_t * __xdata p)
 			if (is_word_x(session, session_id)) {
 				authenticated = 1;
 				last_session_use = now;
-			} else
+			} else {
 				dbg_string("Invalid session cookie!\n");
+			}
 		}
 	}
 	return p;