Fix switch fallthrough: return false before #ifndef _WIN32 backslash case

Author: DeusData

src/foundation/str_util.c

@@ -249,6 +249,7 @@ bool cbm_validate_shell_arg(const char *s) {
         case '`':
         case '\n':
         case '\r':
+            return false;
 #ifndef _WIN32
         case '\\':
             return false;

tests/test_str_util.c

@@ -405,7 +405,12 @@ TEST(validate_shell_arg_carriage_return) {
 }
 
 TEST(validate_shell_arg_backslash) {
+#ifdef _WIN32
+    /* Backslash is allowed on Windows (path separator) */
+    ASSERT_TRUE(cbm_validate_shell_arg("path\\to\\file"));
+#else
     ASSERT_FALSE(cbm_validate_shell_arg("path\\to\\file"));
+#endif
     PASS();
 }