Fix "ignoring return value" complaints from commit 96d1f423f9

The cfbot and some BF animals are complaining about the previous
read_binary_file commit because of ignoring return value of ‘fread’.
So let's make everyone happy by testing the return value even though
not strictly needed.

Reported by Justin Pryzby, and suggested patch by Tom Lane. Backpatched
to v11 same as the previous commit.

Reported-By: Justin Pryzby
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/flat/969b8d82-5bb2-5fa8-4eb1-f0e685c5d736%40joeconway.com
Backpatch-through: 11

src/backend/utils/adt/genfile.c

@@ -167,8 +167,7 @@ read_binary_file(const char *filename, int64 seek_offset, int64 bytes_to_read,
 			{
 				char	rbuf[1]; 
 
-				fread(rbuf, 1, 1, file);
-				if (!feof(file))
+				if (fread(rbuf, 1, 1, file) != 0 || !feof(file))
 					ereport(ERROR,
 							(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 							 errmsg("file length too large")));