debug: coredump: coredump_backend_flash: Update checksum only on success

If the flash stream API fails to write a buffer, exclude it from the
calculated checksum.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>

Author: pdgendt

subsys/debug/coredump/coredump_backend_flash_partition.c

@@ -530,10 +530,6 @@ static void coredump_flash_backend_buffer_output(uint8_t *buf, size_t buflen)
 
 		(void)memmove(tmp_buf, ptr, copy_sz);
 
-		for (i = 0; i < copy_sz; i++) {
-			backend_ctx.checksum += tmp_buf[i];
-		}
-
 		backend_ctx.error = stream_flash_buffered_write(
 					&backend_ctx.stream_ctx,
 					tmp_buf, copy_sz, false);
@@ -542,6 +538,10 @@ static void coredump_flash_backend_buffer_output(uint8_t *buf, size_t buflen)
 			break;
 		}
 
+		for (i = 0; i < copy_sz; i++) {
+			backend_ctx.checksum += tmp_buf[i];
+		}
+
 		ptr += copy_sz;
 		remaining -= copy_sz;
 	}