putenv: handles a string starting with = as error

Change-Id: I1ff288080ac468b5f8a75f2064e68e8086dc7a69
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5539
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/system/libroot/posix/stdlib/env.cpp

@@ -309,7 +309,7 @@ putenv(char *string)
 	char *value = strchr(string, '=');
 	status_t status;
 
-	if (value == NULL) {
+	if (value == NULL || value == string) {
 		__set_errno(B_BAD_VALUE);
 		return -1;
 	}