Fix some compiler warnings

`-Weverything' seems to have found a few problems.

Also enabled clang sanitizers by default.

Makefile

@@ -2,7 +2,8 @@ SHELL = /bin/sh
 CC = clang
 # -Wunused-function is pretty annoying here, as everything is static and not
 # all parts of common.c are used in all the executables
-CFLAGS = -ggdb -Wall -Wextra -std=c99 -Wno-unused-function
+CFLAGS = -std=c99 -Wall -Wextra -Wno-unused-function \
+	-ggdb -fsanitize=address,undefined
 # -lpthread is only there for debugging (gdb & errno)
 LDFLAGS = `pkg-config --libs libssl` -lpthread