feat: Sanitize illegal XML characters from outgoing messages

Filter out control characters (U+0000 to U+001F) from outgoing
messages, as they are illegal in XML 1.0 (except for \t, \n, and \r).
This prevents XMPP servers from closing the connection when such
characters are accidentally or intentionally included in a message.

Fixes: https://github.com/profanity-im/profanity/issues/1437

src/common.c

@@ -418,6 +418,33 @@ utf8_display_len(const char* const str)
     return len;
 }
 
+/**
+ * Removes illegal XML 1.0 characters from a string.
+ *
+ * This function creates a new string that excludes characters in the range
+ * U+0000 to U+001F, except for U+0009 (TAB), U+000A (LF), and U+000D (CR).
+ */
+gchar*
+str_xml_sanitize(const char* const str)
+{
+    if (str == NULL) {
+        return NULL;
+    }
+
+    GString* sanitized = g_string_new_len(NULL, strlen(str));
+    const char* curr = str;
+
+    while (*curr != '\0') {
+        gunichar c = g_utf8_get_char(curr);
+        if ((c >= 0x20) || (c == 0x09) || (c == 0x0A) || (c == 0x0D)) {
+            g_string_append_unichar(sanitized, c);
+        }
+        curr = g_utf8_next_char(curr);
+    }
+
+    return g_string_free(sanitized, FALSE);
+}
+
 char*
 release_get_latest(void)
 {