Bug #58137 char(0) column cause: my_gcvt: Assertion `width > 0 && to != ((void *)0)' failed

mysql-test/r/func_math.result:
  Add test for Bug #58137
mysql-test/t/func_math.test:
  Add test for Bug #58137
sql/field.cc:
  Skip calling my_gcvt() if we are trying to insert a double into a char(0) column.

mysql-test/r/func_math.result

@@ -607,3 +607,12 @@ SELECT floor(log10(format(concat_ws(5445796E25, 5306463, 30837), -358821)))
 as foo;
 foo
 2
+#
+# Bug #58137 char(0) column cause:
+#            my_gcvt: Assertion `width > 0 && to != ((void *)0)' failed
+#
+CREATE TABLE t1(a char(0));
+INSERT INTO t1 (SELECT -pi());
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+DROP TABLE t1;

mysql-test/t/func_math.test

@@ -464,3 +464,11 @@ SELECT -9223372036854775808 MOD -1;
 --echo #
 SELECT floor(log10(format(concat_ws(5445796E25, 5306463, 30837), -358821)))
 as foo;
+
+--echo #
+--echo # Bug #58137 char(0) column cause:
+--echo #            my_gcvt: Assertion `width > 0 && to != ((void *)0)' failed
+--echo #
+CREATE TABLE t1(a char(0));
+INSERT INTO t1 (SELECT -pi());
+DROP TABLE t1;

sql/field.cc

@@ -6327,10 +6327,13 @@ int Field_str::store(double nr)
   ASSERT_COLUMN_MARKED_FOR_WRITE;
   char buff[DOUBLE_TO_STRING_CONVERSION_BUFFER_SIZE];
   uint local_char_length= field_length / charset()->mbmaxlen;
-  size_t length;
-  my_bool error;
+  size_t length= 0;
+  my_bool error= (local_char_length == 0);
 
+  // my_gcvt() requires width > 0, and we may have a CHAR(0) column.
+  if (!error)
     length= my_gcvt(nr, MY_GCVT_ARG_DOUBLE, local_char_length, buff, &error);
+
   if (error)
   {
     if (table->in_use->abort_on_warning)