sort: fix thousands grouping handling on single byte locales

* gl/lib/strnumcmp-in.h (numcompare): After commit v9.0-8-g6cafb122f,
we need to treat characters as signed to avoid invalid comparisons
between negative integers and unsigned characters.
* NEWS: Mention the bug fix.

NEWS

@@ -16,6 +16,11 @@ GNU coreutils NEWS                                    -*- outline -*-
   numfmt options like --suffix no longer have an arbitrary 127-byte limit.
   [bug introduced with numfmt in coreutils-8.21]
 
+  sort again handles thousands grouping characters in single-byte locales
+  where the grouping character is greater than CHAR_MAX.  For e.g. signed
+  character platforms with a 0xA0 (aka &nbsp) grouping character.
+  [bug introduced in coreutils-9.1]
+
   tail no longer mishandles input from files in /proc and /sys file systems,
   on systems with a page size larger than the stdio BUFSIZ.
   [This bug was present in "the beginning".]

gl/lib/strnumcmp-in.h

@@ -114,8 +114,8 @@ static inline int _GL_ATTRIBUTE_PURE
 numcompare (char const *a, char const *b,
             int decimal_point, int thousands_sep)
 {
-  unsigned char tmpa = *a;
-  unsigned char tmpb = *b;
+  char tmpa = *a;
+  char tmpb = *b;
   int tmp;
   size_t log_a;
   size_t log_b;