[libc] Allow openat and creat to return fd 0. (#166466)

Previously, if the `open` or `openat` syscalls returned 0 as a (valid)
file descriptor, the `creat` and `openat` wrappers would erroneously
return -1.

Author: jtstogel

libc/src/fcntl/linux/creat.cpp

@@ -27,11 +27,11 @@ LLVM_LIBC_FUNCTION(int, creat, (const char *path, int mode_flags)) {
       SYS_openat, AT_FDCWD, path, O_CREAT | O_WRONLY | O_TRUNC, mode_flags);
 #endif
 
-  if (fd > 0)
-    return fd;
-
-  libc_errno = -fd;
-  return -1;
+  if (fd < 0) {
+    libc_errno = -fd;
+    return -1;
+  }
+  return fd;
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/fcntl/linux/openat.cpp

@@ -32,11 +32,11 @@ LLVM_LIBC_FUNCTION(int, openat, (int dfd, const char *path, int flags, ...)) {
 
   int fd = LIBC_NAMESPACE::syscall_impl<int>(SYS_openat, dfd, path, flags,
                                              mode_flags);
-  if (fd > 0)
-    return fd;
-
-  libc_errno = -fd;
-  return -1;
+  if (fd < 0) {
+    libc_errno = -fd;
+    return -1;
+  }
+  return fd;
 }
 
 } // namespace LIBC_NAMESPACE_DECL