Ensure we have a valid file handle in tus_file_mmap()

Unfortunately, I was not able to exactly reproduce the bug as
documented, but I am confident that this should fix it.

Fixes #1 (I hope)

src/tus_file.c

@@ -473,7 +473,7 @@ static void
 tus_file_mmap(struct tus_file_core *fcore)
 {
 	struct tus_file_disk *fdisk;
-	int prot = PROT_READ;
+	int fd, prot = PROT_READ;
 	off_t len;
 
 	CHECK_OBJ_NOTNULL(fcore, VMOD_TUS_FILE_CORE_MAGIC);
@@ -489,6 +489,7 @@ tus_file_mmap(struct tus_file_core *fcore)
 	AZ(fcore->ptr);
 	AZ(fcore->len);
 	assert(fcore->ptr_type == TFCP_INVALID);
+	fd = tus_file_open(fcore);
 
 	assert(fdisk->upload_offset > 0);
 	assert(fdisk->upload_length > 0);
@@ -498,11 +499,11 @@ tus_file_mmap(struct tus_file_core *fcore)
 		len += fdisk->upload_length;
 		prot |= PROT_WRITE;
 		// XXX error handling
-		AZ(fallocate(fcore->fd, (int)0, (off_t)0, len));
+		AZ(fallocate(fd, (int)0, (off_t)0, len));
 	}
 
 	fcore->ptr = mmap(NULL, (size_t)fdisk->upload_length, prot,
-	    MAP_SHARED | MAP_NORESERVE, fcore->fd, (off_t)header_size);
+	    MAP_SHARED | MAP_NORESERVE, fd, (off_t)header_size);
 	AN(fcore->ptr);
 	fcore->ptr_type = (prot & PROT_WRITE) ?
 		TFCP_MMAP_RW : TFCP_MMAP_RO;