Fix GCC build warning in NetBSD 10

The warning message is
"array subscript has type 'char' [-Wchar-subscripts]"

Fix this by casting to 'unsigned char' before passing any character to a
`<ctype.h>` function.

Also add an assertion to RichString_writeFromAscii() to ensure the
characters in the string are all in ASCII range.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Author: Explorer09

RichString.c

@@ -9,7 +9,9 @@ in the source distribution for its full text.
 
 #include "RichString.h"
 
+#include <assert.h>
 #include <ctype.h>
+#include <limits.h> // IWYU pragma: keep
 #include <stdlib.h>
 #include <string.h>
 
@@ -144,7 +146,8 @@ static inline int RichString_writeFromAscii(RichString* this, int attrs, const c
    int newLen = from + len;
    RichString_setLen(this, newLen);
    for (int i = from, j = 0; i < newLen; i++, j++) {
-      this->chptr[i] = (CharType) { .attr = attrs & 0xffffff, .chars = { (isprint(data[j]) ? data[j] : L'\xFFFD') } };
+      assert((unsigned char)data[j] <= SCHAR_MAX);
+      this->chptr[i] = (CharType) { .attr = attrs & 0xffffff, .chars = { (isprint((unsigned char)data[j]) ? data[j] : L'\xFFFD') } };
    }
 
    return len;

Settings.c

@@ -233,7 +233,7 @@ static const char* toFieldName(Hashtable* columns, int id, bool* enabled) {
 }
 
 static int toFieldIndex(Hashtable* columns, const char* str) {
-   if (isdigit(str[0])) {
+   if (isdigit((unsigned char)str[0])) {
       // This "+1" is for compatibility with the older enum format.
       int id = atoi(str) + 1;
       if (toFieldName(columns, id, NULL)) {