unix: fix various memory leaks and undef behavior

Kindly suggested by `cppcheck`.

src/unix/darwin.c

@@ -75,7 +75,7 @@ int uv_exepath(char* buffer, size_t* size) {
   result = _NSGetExecutablePath(buffer, &usize);
   if (result) return result;
 
-  path = (char*)malloc(2 * PATH_MAX);
+  path = malloc(2 * PATH_MAX);
   fullpath = realpath(buffer, path);
   if (fullpath == NULL) {
     SAVE_ERRNO(free(path));

src/unix/fsevents.c

@@ -590,7 +590,7 @@ static int uv__fsevents_loop_init(uv_loop_t* loop) {
 
   err = uv_mutex_init(&loop->cf_mutex);
   if (err)
-    return err;
+    goto fail_mutex_init;
 
   err = uv_sem_init(&loop->cf_sem, 0);
   if (err)
@@ -660,6 +660,8 @@ fail_fsevent_sem_init:
 
 fail_sem_init:
   uv_mutex_destroy(&loop->cf_mutex);
+
+fail_mutex_init:
   free(state);
   return err;
 }

src/unix/signal.c

@@ -348,6 +348,7 @@ static void uv__signal_event(uv_loop_t* loop,
   int r;
 
   bytes = 0;
+  end = 0;
 
   do {
     r = read(loop->signal_pipefd[0], buf + bytes, sizeof(buf) - bytes);