Always copy macro source when expanding it

- A macro can undefine itself, and unless we grab a copy of it we'll
  end up accessing already freed memory. Fixes a regression from
  commit ebc4ceaaeb which assumed
  a copy is not always needed.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>

rpmio/macro.c

@@ -1022,12 +1022,12 @@ expandMacro(MacroBuf mb, const char *src, size_t slen)
     char *source = NULL;
 
     /* Handle non-terminated substrings by creating a terminated copy */
-    if (slen > 0) {
-	source = xmalloc(slen + 1);
-	strncpy(source, src, slen);
-	source[slen] = '\0';
-	s = source;
-    }
+    if (!slen)
+	slen = strlen(src);
+    source = xmalloc(slen + 1);
+    strncpy(source, src, slen);
+    source[slen] = '\0';
+    s = source;
 
     if (mb->buf == NULL) {
 	size_t blen = MACROBUFSIZ + strlen(s);