py/mpz: Skip separators when running out of digits to print.

This commit fixes the addition of a stray separator before the number
when printing an MPZ-backed integer and the first group is three digits
long.

This fixes #8984.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

py/mpz.c

@@ -1717,7 +1717,7 @@ size_t mpz_as_str_inpl(const mpz_t *i, unsigned int base, const char *prefix, ch
                 break;
             }
         }
-        if (comma && (s - last_comma) == 3) {
+        if (!done && comma && (s - last_comma) == 3) {
             *s++ = comma;
             last_comma = s;
         }

tests/basics/string_format_intbig.py

@@ -0,0 +1,15 @@
+# basic functionality test for {} format string using large integers
+
+
+def test(fmt, *args):
+    print("{:8s}".format(fmt) + ">" + fmt.format(*args) + "<")
+
+
+# Separator formatter
+
+test("{:,}", 123_456_789_012_345_678_901_234_567)
+test("{:,}", 23_456_789_012_345_678_901_234_567)
+test("{:,}", 3_456_789_012_345_678_901_234_567)
+test("{:,}", -123_456_789_012_345_678_901_234_567)
+test("{:,}", -23_456_789_012_345_678_901_234_567)
+test("{:,}", -3_456_789_012_345_678_901_234_567)