Fix #7047 Safely handle setlocale (#8735)

* fix #7074 Safely handle setlocale

`setlocale` returns a pointer to a buffer containing the current locale name.
This needs to be copied into a `std::string` or it will be overwritten by the next call.

Trying to call `setlocale` with a non-null, invalid pointer can have unpredictable results, such as
```
[ RUN      ] StringPrintfTest.Multibyte
minkernel\crts\ucrt\src\appcrt\convert\mbstowcs.cpp(246) : Assertion failed: (pwcs == nullptr && sizeInWords == 0) || (pwcs != nullptr && sizeInWords > 0)
```

`setlocale` can also return a `nullptr` if it fails, but we assert against that.

* stringprintf_unittest.cc: Replace `new char[n+1]` with `std::array`

Prefer safer alternative to naked pointers.

This is a follow-up to 1dd313c

src/google/protobuf/stubs/stringprintf_unittest.cc

@@ -32,6 +32,7 @@
 
 #include <google/protobuf/stubs/stringprintf.h>
 
+#include <array>
 #include <cerrno>
 #include <string>
 
@@ -91,7 +92,9 @@ TEST(StringPrintfTest, Multibyte) {
   // out of memory while trying to determine destination buffer size.
   // see b/4194543.
 
-  char* old_locale = setlocale(LC_CTYPE, nullptr);
+  char* old_locale_c = setlocale(LC_CTYPE, nullptr);
+  ASSERT_TRUE(old_locale_c != nullptr);
+  std::string old_locale = old_locale_c;
   // Push locale with multibyte mode
   setlocale(LC_CTYPE, "en_US.utf8");
 
@@ -106,24 +109,25 @@ TEST(StringPrintfTest, Multibyte) {
 
   // Repeat with longer string, to make sure that the dynamically
   // allocated path in StringAppendV is handled correctly.
-  int n = 2048;
-  char* buf = new char[n+1];
-  memset(buf, ' ', n-3);
-  memcpy(buf + n - 3, kInvalidCodePoint, 4);
-  value =  StringPrintf("%.*s", n, buf);
+  const size_t n = 2048;
+  std::array<char, n+1> buf;
+  memset(&buf[0], ' ', n-3);
+  memcpy(&buf[0] + n - 3, kInvalidCodePoint, 4);
+  value =  StringPrintf("%.*s", n, &buf[0]);
   // See GRTEv2 vs. GRTEv3 comment above.
-  EXPECT_TRUE(value.empty() || value == buf);
-  delete[] buf;
+  EXPECT_TRUE(value.empty() || value == &buf[0]);
 
-  setlocale(LC_CTYPE, old_locale);
+  setlocale(LC_CTYPE, old_locale.c_str());
 }
 
 TEST(StringPrintfTest, NoMultibyte) {
   // No multibyte handling, but the string contains funny chars.
-  char* old_locale = setlocale(LC_CTYPE, nullptr);
+  char* old_locale_c = setlocale(LC_CTYPE, nullptr);
+  ASSERT_TRUE(old_locale_c != nullptr);
+  std::string old_locale = old_locale_c;
   setlocale(LC_CTYPE, "POSIX");
   std::string value = StringPrintf("%.*s", 3, "\375\067s");
-  setlocale(LC_CTYPE, old_locale);
+  setlocale(LC_CTYPE, old_locale.c_str());
   EXPECT_EQ("\375\067s", value);
 }