pem.c: fixed warning C6269: possible incorrect order of operations

src/pem.c

@@ -89,7 +89,7 @@ readline_memory(char *line, size_t line_size,
     }
 
     line[len] = '\0';
-    *filedata_offset++;
+    *filedata_offset += 1;
 
     return 0;
 }