http.c: Fix wrong check

p->f is a FILE *, so it is invalid to check against negative values.

This bug was introduced when p->fd, a file descriptor, was replaced with
p->f, a FILE *, by the following commit:

commit b0accd099fa8c5110d4c3c68830ad6fd810ca3ec
Author: Xavier Del Campo Romero <xavi.dcr@tutanota.com>
Date:   Fri Nov 24 00:52:50 2023 +0100

    http.c: Unify read operations

1 files changed, 1 insertions, 1 deletions

diff --git a/http.c b/http.c
index 19a1b93..cf2dd8c 100644
--- a/http.c
+++ b/http.c
@@ -493,7 +493,7 @@ static void ctx_free(struct ctx *const c)
     {
         struct put *const p = &c->u.put;
 
-        if (p->f >= 0 && fclose(p->f))
+        if (p->f && fclose(p->f))
             fprintf(stderr, "%s: fclose(3) p->f: %s\n",
                 __func__, strerror(errno));