Ensure custom preferences path ends with path separator

Example:
  Given: FSO_PREFERENCES_PATH='./config' fs2_open
  Path becomes: './config/'

code/osapi/osapi.cpp

@@ -49,6 +49,15 @@ namespace
 			auto envPreferencesPath = getenv("FSO_PREFERENCES_PATH");
 			if (envPreferencesPath != nullptr && strlen(envPreferencesPath) > 0) {
 				preferencesPath = envPreferencesPath;
+
+				// Ensure path ends with a path separator (slash)
+				size_t length = strlen(preferencesPath);
+				if (preferencesPath[length - 1] != DIR_SEPARATOR_CHAR) {
+					char* fixedPath = new char[length + 2]; // +1 for DIR_SEPARATOR, +1 for '\0'
+					strcpy(fixedPath, preferencesPath);
+					strcat(fixedPath, DIR_SEPARATOR_STR);
+					preferencesPath = fixedPath;
+				}
 			}
 			else {
 				preferencesPath = SDL_GetPrefPath(ORGANIZATION_NAME, APPLICATION_NAME);