menu/rgui: convert OSK fallback messagebox build to strlcpy_append

The fallback path in rgui_render_osk that triggers when the on-screen
keyboard cannot fit on the framebuffer was building its message via
the unsafe `*pos += strlcpy(buf + *pos, src, len - *pos)` pattern.
The construction was sound here because msg is large
(NAME_MAX_LENGTH) and inputs are short, but the same string-build
shape is what strlcpy_append exists to replace -- it's bound-checked,
short-circuits cleanly on truncation, and avoids the off-by-one
arithmetic around the embedded '\n'.

Mirrors materialui_render's identical construction at materialui.c
line ~8091, which was already converted in the strlcpy_append
chain-conversion sweeps (commits 25ade82 / e446242 / 78c52ab).

Author: LibretroAdmin

menu/drivers/rgui.c

@@ -4786,12 +4786,10 @@ static void rgui_render_osk(
        * If OSK cannot physically fit on the screen,
        * fallback to old style 'message box' implementation */
       char msg[NAME_MAX_LENGTH];
-      size_t _len = strlcpy(msg, input_label, sizeof(msg) - 2);
-      msg[  _len] = '\n';
-      msg[++_len] = '\0';
-      strlcpy(msg       + _len,
-            input_str,
-            sizeof(msg) - _len);
+      size_t _len = 0;
+      strlcpy_append(msg, sizeof(msg), &_len, input_label);
+      strlcpy_append(msg, sizeof(msg), &_len, "\n");
+      strlcpy_append(msg, sizeof(msg), &_len, input_str);
       rgui_render_messagebox(rgui, msg, fb_width, fb_height);
       return;
    }