fix: add NULL check for c->funcs before dereferencing in redisReconnect

redisReconnect dereferences c->funcs->free_privctx without checking
if c->funcs is NULL, even though the very next block (line 780)
correctly guards c->funcs before accessing c->funcs->close.

This can cause a NULL pointer dereference crash if redisReconnect
is called on a context where funcs has not been set.

Fixes #1315

Author: Charlie Tonneslan

hiredis.c

@@ -772,7 +772,7 @@ int redisReconnect(redisContext *c) {
     c->err = 0;
     memset(c->errstr, '\0', strlen(c->errstr));
 
-    if (c->privctx && c->funcs->free_privctx) {
+    if (c->privctx && c->funcs && c->funcs->free_privctx) {
         c->funcs->free_privctx(c->privctx);
         c->privctx = NULL;
     }