Fix commit 2358a19

Spotted by Daniel Dragan. We use GetEnvironmentStringsW() (deliberately so:
see commit 4f46e52), so we must use FreeEnvironmentStringsW() to match.

(cherry picked from commit a6abe94)

win32/win32.c

@@ -1774,7 +1774,7 @@ win32_getenvironmentstrings(void)
     WideCharToMultiByte(CP_ACP, WC_NO_BEST_FIT_CHARS, lpWStr, wenvstrings_len, lpStr, 
                         aenvstrings_len, NULL, NULL);
 
-    FreeEnvironmentStrings(lpWStr);
+    FreeEnvironmentStringsW(lpWStr);
 
     return(lpStr);
 }