gtk-demo/main: Suppress implicit fallthru warning

Comments matched to reassure the compiler that fallthrough is
intentional are supposed to precede the case or default keywords, at
least in GCC, so the one here did not suppress the warning with GCC. We
can just the if condition and put the comment at the end to solve that.

https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

demos/gtk-demo/main.c

@@ -831,15 +831,14 @@ load_file (const gchar *demoname,
           /* Skipping blank lines */
           while (g_ascii_isspace (*p))
             p++;
-          if (*p)
-            {
-              p = lines[i];
-              state++;
-              /* Fall through */
-            }
-          else
+
+          if (!*p)
             break;
 
+          p = lines[i];
+          state++;
+          /* Fall through */
+
         case 3:
           /* Reading program body */
           gtk_text_buffer_insert (source_buffer, &start, p, -1);