check return value of malloc_read_fd

in case `malloc_read_fd` returns a negative error number, the result
would afterwards be casted to an unsigned size_t, and may have
theoretically caused an out-of-bounds memory access in the following
`strncmp` call.

src/pages.c

@@ -567,6 +567,10 @@ init_thp_state(void) {
 	close(fd);
 #endif
 
+        if (nread < 0) {
+		goto label_error; 
+        }
+
 	if (strncmp(buf, sys_state_madvise, (size_t)nread) == 0) {
 		init_system_thp_mode = thp_mode_default;
 	} else if (strncmp(buf, sys_state_always, (size_t)nread) == 0) {