MDEV-8407 Numeric errors, server crash with COLUMN_JSON() on DECIMAL with precision > 40

In fact it was error in decimal library (incorrect processing of buffer overflow) invisible from other server parts because of buffer allocation and precision tests.

strings/decimal.c

@@ -383,7 +383,8 @@ int decimal2string(const decimal_t *from, char *to, int *to_len,
     }
     else
       frac-=j;
-    len= from->sign + intg_len + test(frac) + frac_len;
+    frac_len= frac;
+    len= from->sign + intg_len + test(frac) + frac;
   }
   *to_len=len;
   s[len]=0;

unittest/my_decimal/my_decimal-t.cc

@@ -61,12 +61,42 @@ test_copy_and_compare()
 
 }
 
+static int
+test_decimal2string()
+{
+  decimal_t d1;
+  decimal_digit_t buffer[DECIMAL_BUFF_LENGTH+2];
+  char *str_end;
+  const char strnum[]= "0.1234567890123456789012345678901234567890123467";
+  char strbuff[50];
+  int len= 40;
+  int i;
+
+  bzero(strbuff, sizeof(strbuff));
+  str_end= (char *)(strnum + (sizeof(strnum) - 1));
+
+  d1.len= DECIMAL_BUFF_LENGTH + 2;
+  d1.buf= buffer;
+
+  string2decimal(strnum, &d1, &str_end);
+  decimal2string(&d1, strbuff, &len, 0, 0, 'X');
+
+  /* last digit is not checked due to possible rounding */
+  for (i= 0; i < 38 && strbuff[i] == strnum[i]; i++);
+  ok(i == 38, "Number");
+  for (i= 39; i < 50 && strbuff[i] == 0; i++);
+  ok(i == 50, "No overrun");
+
+  return 0;
+
+}
 int main()
 {
-  plan(13);
+  plan(15);
   diag("Testing my_decimal constructor and assignment operators");
 
   test_copy_and_compare();
+  test_decimal2string();
 
   return exit_status();
 }