Add error handling for failing fstat() calls in copy.c.

These calls are pretty much guaranteed not to fail unless something
has gone horribly wrong, and even in that case we'd just error out a
short time later.  But since several code checkers complain about the
missing check it seems worthwile to fix it nonetheless.

Pointed out by Coverity.

src/backend/commands/copy.c

@@ -1707,7 +1707,9 @@ BeginCopyTo(Relation rel,
 						 errmsg("could not open file \"%s\" for writing: %m",
 								cstate->filename)));
 
-			fstat(fileno(cstate->copy_file), &st);
+			if (fstat(fileno(cstate->copy_file), &st))
+				elog(ERROR, "could not stat file \"%s\": %m", cstate->filename);
+
 			if (S_ISDIR(st.st_mode))
 				ereport(ERROR,
 						(errcode(ERRCODE_WRONG_OBJECT_TYPE),
@@ -2718,7 +2720,9 @@ BeginCopyFrom(Relation rel,
 						 errmsg("could not open file \"%s\" for reading: %m",
 								cstate->filename)));
 
-			fstat(fileno(cstate->copy_file), &st);
+			if (fstat(fileno(cstate->copy_file), &st))
+				elog(ERROR, "could not stat file \"%s\": %m", cstate->filename);
+
 			if (S_ISDIR(st.st_mode))
 				ereport(ERROR,
 						(errcode(ERRCODE_WRONG_OBJECT_TYPE),