[Bug]: Crash when TEST_PREMATURE_EXIT_FILE points to invalid path

### Describe the issue

In ScopedPrematureExitFile constructor, when posix::FOpen() fails to
create the premature exit file, it returns nullptr. The code then
calls fwrite() and fclose() on this nullptr, causing a crash.

The comment states "I/O errors are ignored", but the code does not
actually handle the failure case.

Location: googletest/src/gtest.cc lines 5148-5150

Current code:
```
FILE* pfile = posix::FOpen(premature_exit_filepath_.c_str(), "w");
fwrite("0", 1, 1, pfile); // crashes if pfile is nullptr
fclose(pfile); // crashes if pfile is nullptr
```
Other FOpen usages in the codebase properly check for nullptr before
use.

### Steps to reproduce the problem

1. Build GoogleTest
```
mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Debug
make -j4
```

2. Create a simple test program (test_simple.cpp)
```
#include

TEST(SimpleTest, Pass) {
EXPECT_EQ(1, 1);
}

int main(int argc, char **argv) {
testing::InitGoogleTest(&argc, argv);
return RUN_ALL_TESTS();
}
```
3. Compile the test program
```
g++ test_simple.cpp -o test_bug \
-std=c++17 \
-I../googletest/include \
-L./lib -lgtest -lgtest_main -pthread
```
4. Run with invalid path
`TEST_PREMATURE_EXIT_FILE=/nonexistent/path/file.txt ./test_bug`
5. Result: Program crashes with segmentation fault
zsh: segmentation fault

### What version of GoogleTest are you using?

`85087857ad10bd407cd6ed2f52f7ea9752db621f`

### What operating system and version are you using?

macOS 13.4

### What compiler and version are you using?

```
Apple clang version 14.0.3 (clang-1403.0.22.14.1)
Target: arm64-apple-darwin22.5.0
```

### What build system are you using?

CMake 4.2.1

### Additional context

```
FILE* pfile = posix::FOpen(premature_exit_filepath_.c_str(), "w");
if (pfile != nullptr) {
fwrite("0", 1, 1, pfile);
fclose(pfile);
}
```

This is consistent with other FOpen usages in the codebase (e.g.,
line 5899).

I have verified this fix resolves the crash.

Start in googletest/src/gtest.cc around lines 5148-5150 and compare the other FOpen usage around line 5899. Reproduce the failure with TEST_PREMATURE_EXIT_FILE pointing to an invalid path, then verify the program no longer crashes and that the existing build commands still pass.

Written by the indexing model from the issue text.