Only convert the result of XLookupString() if it's not already UTF-8

Fixes https://github.com/libsdl-org/SDL/issues/7766

src/stdlib/SDL_string.c

@@ -36,7 +36,7 @@
 #define UTF8_IsLeadByte(c)     ((c) >= 0xC0 && (c) <= 0xF4)
 #define UTF8_IsTrailingByte(c) ((c) >= 0x80 && (c) <= 0xBF)
 
-static unsigned UTF8_GetTrailingBytes(unsigned char c)
+static size_t UTF8_GetTrailingBytes(unsigned char c)
 {
     if (c >= 0xC0 && c <= 0xDF) {
         return 1;
@@ -676,6 +676,42 @@ size_t SDL_utf8strnlen(const char *str, size_t bytes)
     return retval;
 }
 
+SDL_bool SDL_utf8valid(const char *str, size_t bytes)
+{
+    while (*str && bytes > 0) {
+        Uint8 ch = (Uint8)*str;
+
+        if (ch <= 0x7F) {
+            ++str;
+            --bytes;
+            continue;
+        }
+
+        if (UTF8_IsLeadByte(ch)) {
+            size_t left = UTF8_GetTrailingBytes(ch);
+            if (bytes < left) {
+                return SDL_FALSE;
+            }
+
+            ++str;
+            --bytes;
+
+            while (left-- > 0) {
+                ch = (Uint8)*str;
+                if (!UTF8_IsTrailingByte(ch)) {
+                    return SDL_FALSE;
+                }
+
+                ++str;
+                --bytes;
+            }
+        } else {
+            return SDL_FALSE;
+        }
+    }
+    return SDL_TRUE;
+}
+
 size_t SDL_strlcat(SDL_INOUT_Z_CAP(maxlen) char *dst, const char *src, size_t maxlen)
 {
 #ifdef HAVE_STRLCAT