LibC: Return EINVAL from unsetenv() if input is empty or contains "="

Author: https://github.com/tcl3
Commit: b9e7998b53
Pull-request: https://github.com/SerenityOS/serenity/pull/19630

Userland/Libraries/LibC/stdlib.cpp

@@ -431,6 +431,11 @@ char* secure_getenv(char const* name)
 int unsetenv(char const* name)
 {
     auto new_var_len = strlen(name);
+    if (new_var_len == 0 || strchr(name, '=')) {
+        errno = EINVAL;
+        return -1;
+    }
+
     size_t environ_size = 0;
     int skip = -1;