fix undefined behavior triggered by fn inline test

LLVM destroys the string that we use to test if LLVM deleted the
inlined function.

Also fixed forgetting to initialize a buffer in std lib path detection.

src/main.cpp

@@ -179,6 +179,7 @@ static int find_zig_lib_dir(Buf *out_path) {
     Buf self_exe_path = BUF_INIT;
     if (!(err = os_self_exe_path(&self_exe_path))) {
         Buf *cur_path = &self_exe_path;
+        buf_resize(cur_path, 0);
 
         for (;;) {
             Buf *test_dir = buf_alloc();