Fix #4890

`strings.to_cstring` previously would not check if the buffer could
handle the extra null byte and could lead to segmentation violations
when using the resulting string in an API expecting the terminator.

core/strings/builder.odin

@@ -288,18 +288,41 @@ to_string :: proc(b: Builder) -> (res: string) {
 /*
 Appends a trailing null byte after the end of the current Builder byte buffer and then casts it to a cstring
 
+NOTE: This procedure will not check if the backing buffer has enough space to include the extra null byte.
+
 Inputs:
 - b: A pointer to builder
 
 Returns:
 - res: A cstring of the Builder's buffer
 */
-to_cstring :: proc(b: ^Builder) -> (res: cstring) {
+unsafe_to_cstring :: proc(b: ^Builder) -> (res: cstring) {
 	append(&b.buf, 0)
 	pop(&b.buf)
 	return cstring(raw_data(b.buf))
 }
 /*
+Appends a trailing null byte after the end of the current Builder byte buffer and then casts it to a cstring
+
+Inputs:
+- b: A pointer to builder
+
+Returns:
+- res: A cstring of the Builder's buffer upon success
+- err: An optional allocator error if one occured, `nil` otherwise
+*/
+to_cstring :: proc(b: ^Builder) -> (res: cstring, err: mem.Allocator_Error) {
+	n := append(&b.buf, 0) or_return
+	if n != 1 {
+		return nil, .Out_Of_Memory
+	}
+	pop(&b.buf)
+	#no_bounds_check {
+		assert(b.buf[len(b.buf)] == 0)
+	}
+	return cstring(raw_data(b.buf)), nil
+}
+/*
 Returns the length of the Builder's buffer, in bytes
 
 Inputs: