fix: Free home-screen plugin tagline before reallocating

get_appname_and_tagline() allocates g_tag_line on every home-screen
transition where the caller is a plugin, but nothing freed the prior
buffer first ? the comment at the allocation point even acknowledged
"will never be deallocated". Each return to the home screen during a
plugin-driven session (post-tx, post-cancel, multi-flow plugins)
therefore leaked roughly the plugin name length plus the tagline
template into the shared app-memory pool, slowly starving the rest
of the session.

Release the previous allocation with APP_MEM_FREE_AND_NULL() before
the next APP_MEM_CALLOC() so the pool only ever holds one live
tagline buffer.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: cedelavergne-ledger

src/nbgl/ui_home.c

@@ -237,7 +237,13 @@ static void get_appname_and_tagline(const char **appname, const char **tagline)
                 return;
             }
             size_t line_len = 1 + strlen(FORMAT_PLUGIN) + name_len;
-            // Allocate the buffer - will never be deallocated...
+            // Free any tagline left over from a previous home transition
+            // before allocating a new one; without this, repeated returns
+            // to the home screen in the same session accumulate orphaned
+            // buffers in the app-memory pool.
+            if (g_tag_line != NULL) {
+                APP_MEM_FREE_AND_NULL((void **) &g_tag_line);
+            }
             if (APP_MEM_CALLOC((void **) &g_tag_line, line_len) == true) {
                 snprintf(g_tag_line, line_len, FORMAT_PLUGIN, *appname);
                 *tagline = g_tag_line;