bootctl: tweak DOS header magic check

Read the magic first, try reading the full DOS exe header only in case
the magic check succeeds.

This avoids throwing an header read error on small dummy files as used
by test-kernel-install.

(cherry picked from commit 78088b8)

Related: RHEL-16354

src/boot/bootctl-uki.c

@@ -30,11 +30,16 @@ static int pe_sections(FILE *uki, struct PeSectionHeader **ret, size_t *ret_n) {
         assert(ret_n);
 
         items = fread(&dos, 1, sizeof(dos), uki);
-        if (items != sizeof(dos))
-                return log_error_errno(SYNTHETIC_ERRNO(EIO), "DOS header read error");
+        if (items < sizeof(dos.Magic))
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "File is smaller than DOS magic (got %"PRIu64" of %zu bytes)",
+                                       items, sizeof(dos.Magic));
         if (memcmp(dos.Magic, dos_file_magic, sizeof(dos_file_magic)) != 0)
                 goto no_sections;
 
+        if (items != sizeof(dos))
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "File is smaller than DOS header (got %"PRIu64" of %zu bytes)",
+                                       items, sizeof(dos));
+
         if (fseek(uki, le32toh(dos.ExeHeader), SEEK_SET) < 0)
                 return log_error_errno(errno, "seek to PE header");