bug#34792: 27.0.50; format %x and %X don't print as unsigned

[Ivan Andrus]

The following snippets return strings with negative signs, but the
docstring of format says they should be unsigned.  I get the same
results from emacs -Q as well

(format "%s_%X%X" "bob" (random) (random))
;; "bob_143E5019B284E085-3D71BF28F82AFD0"
(format "%s_%x%x" "bob" (random) (random))
;; "bob_114d97ac7a6b2a84-15fcceadc374b70e"


-Ivan

In GNU Emacs 27.0.50 (build 7, x86_64-apple-darwin18.2.0, NS appkit-1671.20 
Version 10.14.3 (Build 18D109))
of 2019-03-08 built on iandrus-macOS
Repository revision: dd77d672d8a9529ce8d4a0649be1b44bbd1a03b1
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.3

Recent messages:
Mark set
Occurrence 1/2 in buffer
find-function-library: Symbol’s function definition is void: ## [2 times]
C-h C-u is undefined
Mark saved where search started
Mark set [2 times]
"bob_-3D1C1371061ABEA-1D876D356D7D25DF"
Saving file /Users/iandrus/.emacs.d/init.el...
Wrote /Users/iandrus/.emacs.d/init.el
Undo! [2 times]

Configured using:
'configure
PKG_CONFIG_PATH=/opt/X11/lib/pkgconfig:/usr/local/opt/libxml2/lib/pkgconfig
--with-sound=yes --with-ns --with-modules --with-file-notification=yes
--enable-gcc-warnings=warn-only --with-xpm --with-jpeg --with-tiff
--with-gif --with-png --with-rsvg --with-xml2 --with-imagemagick
--with-json --with-xft --with-libotf --with-gnutls=no --with-makeinfo
--with-libgmp'

Configured features:
RSVG IMAGEMAGICK GLIB NOTIFY KQUEUE ACL LIBXML2 ZLIB TOOLKIT_SCROLL_BARS
XIM NS MODULES THREADS PDUMPER GMP

Important settings:
 value of $LANG: en_US.UTF-8
 locale-coding-system: utf-8-unix

Major mode: ELisp:Dyn