fix textedit_sample a bit

tests/textedit_sample.c

@@ -32,16 +32,17 @@ void layout_func(StbTexteditRow *row, STB_TEXTEDIT_STRING *str, int start_i)
 
 int delete_chars(STB_TEXTEDIT_STRING *str, int pos, int num)
 {
-   memmove(&str->string[pos], &str->string[pos+num], str->stringlen - (pos-num));
+   memmove(&str->string[pos], &str->string[pos+num], str->stringlen - pos - num);
    str->stringlen -= num;
    return 1; // always succeeds
 }
 
 int insert_chars(STB_TEXTEDIT_STRING *str, int pos, STB_TEXTEDIT_CHARTYPE *newtext, int num)
 {
-   str->string = realloc(str, str->stringlen + num);
+   str->string = realloc(str->string, str->stringlen + num);
    memmove(&str->string[pos+num], &str->string[pos], str->stringlen - pos);
    memcpy(&str->string[pos], newtext, num);
+   str->stringlen += num;
    return 1; // always succeeds
 }