Bug#30394: Empty HOME environment variable causes several utilities to crash

Tilde expansion could fail when it was to expand to an empty string (such as
when HOME is set to an empty string), especially on systems where size_t is
unsigned.

mysys/mf_pack.c

@@ -343,7 +343,7 @@ uint unpack_dirname(my_string to, const char *from)
       length-=(uint) (suffix-buff)-1;
       if (length+(h_length= (uint) strlen(tilde_expansion)) <= FN_REFLEN)
       {
-	if (tilde_expansion[h_length-1] == FN_LIBCHAR)
+	if ((h_length > 0) && (tilde_expansion[h_length-1] == FN_LIBCHAR))
 	  h_length--;
 	if (buff+h_length < suffix)
 	  bmove(buff+h_length,suffix,length);