Fix possible buffer overflows on 32 bit systems

Comparing pointers after adding lengths is dangerous, since the length can overflow the pointer, so that the comparison leads to wrong results.
Comparing lengths only fixes this issue.

This lead to segfault in the following spec on x86:

	it "should raise an error at grabage COPY format" do
		expect{ decoder.decode("123\t \0\\\t\\") }
			.to raise_error(ArgumentError, /premature.*at position: 7$/)
	end

ext/pg_copy_coder.c

@@ -795,26 +795,26 @@ pg_bin_dec_copy_row(t_pg_coder *conv, const char *input_line, int len, int _tupl
 	cur_ptr = input_line;
 	line_end_ptr = input_line + len;
 
-	if (cur_ptr + 11 <= line_end_ptr && memcmp(cur_ptr, BinarySignature, 11) == 0){
+	if (line_end_ptr - cur_ptr >= 11 && memcmp(cur_ptr, BinarySignature, 11) == 0){
 		/* binary COPY header signature detected -> just drop it */
 		int ext_bytes;
 		cur_ptr += 11;
 
 		/* read flags */
-		if (cur_ptr + 4 > line_end_ptr) goto length_error;
+		if (line_end_ptr - cur_ptr < 4 ) goto length_error;
 		cur_ptr += 4;
 
 		/* read header extensions */
-		if (cur_ptr + 4 > line_end_ptr) goto length_error;
+		if (line_end_ptr - cur_ptr < 4 ) goto length_error;
 		ext_bytes = read_nbo32(cur_ptr);
 		if (ext_bytes < 0) goto length_error;
 		cur_ptr += 4;
-		if (cur_ptr + ext_bytes > line_end_ptr) goto length_error;
+		if (line_end_ptr - cur_ptr < ext_bytes ) goto length_error;
 		cur_ptr += ext_bytes;
 	}
 
 	/* read row header */
-	if (cur_ptr + 2 > line_end_ptr) goto length_error;
+	if (line_end_ptr - cur_ptr < 2 ) goto length_error;
 	nfields = read_nbo16(cur_ptr);
 	cur_ptr += 2;
 
@@ -830,7 +830,7 @@ pg_bin_dec_copy_row(t_pg_coder *conv, const char *input_line, int len, int _tupl
 			VALUE field_value;
 
 			/* read field size */
-			if (cur_ptr + 4 > line_end_ptr) goto length_error;
+			if (line_end_ptr - cur_ptr < 4 ) goto length_error;
 			input_len = read_nbo32(cur_ptr);
 			cur_ptr += 4;
 
@@ -839,7 +839,7 @@ pg_bin_dec_copy_row(t_pg_coder *conv, const char *input_line, int len, int _tupl
 				/* NULL indicator */
 				rb_ary_push(array, Qnil);
 			} else {
-				if (cur_ptr + input_len > line_end_ptr) goto length_error;
+				if (line_end_ptr - cur_ptr < input_len ) goto length_error;
 
 				/* copy input data to field_str */
 				PG_RB_STR_ENSURE_CAPA( field_str, input_len, output_ptr, end_capa_ptr );