auto merge of #10348 : ksh8281/rust/fix_android_timegm, r=yichoi

fix timegm bug on android
TZ restore not correctly before.
and it cause test case fail (time::tests::run_tests::test_convertions)

with @sammykim

Author: bors

src/rt/rust_builtin.cpp

@@ -43,12 +43,15 @@ timegm(struct tm *tm)
     char *tz;
 
     tz = getenv("TZ");
+    if (tz)
+        tz = strdup(tz);
     setenv("TZ", "", 1);
     tzset();
     ret = mktime(tm);
-    if (tz)
+    if (tz) {
         setenv("TZ", tz, 1);
-    else
+        free(tz);
+    } else
         unsetenv("TZ");
     tzset();
     return ret;