Add a missing const.

Similarly to commit 1c4f12252, this was one of those errors that C
can't catch because of the const-removing prototype of memchr. But
when you memchr() a const string, the value you get back is
_semantically_ a pointer to const, and should be assigned into a
variable of that type.

be_misc.c

@@ -64,7 +64,8 @@ void log_proxy_stderr(Plug plug, bufchain *buf, const void *vdata, int len)
     const char *data = (const char *)vdata;
     int pos = 0;
     int msglen;
-    char *nlpos, *msg, *fullmsg;
+    const char *nlpos;
+    char *msg, *fullmsg;
 
     /*
      * This helper function allows us to collect the data written to a