Merge in release25-maint r60793:

 Added checks for integer overflows, contributed by Google. Some are
 only available if asserts are left in the code, in cases where they
 can't be triggered from Python code.
diff --git a/Modules/_csv.c b/Modules/_csv.c
index c628927..a5787d3 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -559,6 +559,10 @@
 		self->field = PyMem_Malloc(self->field_size);
 	}
 	else {
+		if (self->field_size > INT_MAX / 2) {
+			PyErr_NoMemory();
+			return 0;
+		} 
 		self->field_size *= 2;
 		self->field = PyMem_Realloc(self->field, self->field_size);
 	}
@@ -1053,6 +1057,12 @@
 static int
 join_check_rec_size(WriterObj *self, int rec_len)
 {
+
+	if (rec_len < 0 || rec_len > INT_MAX - MEM_INCR) {
+		PyErr_NoMemory();
+		return 0;
+	}
+
 	if (rec_len > self->rec_size) {
 		if (self->rec_size == 0) {
 			self->rec_size = (rec_len / MEM_INCR + 1) * MEM_INCR;