Use temp pointer for realloc()

Use temp pointer for realloc()

- Fixes the following Cppcheck 1.89 errors:
  [src/rrd_cgi.c:295] (error) Common realloc mistake:'varheap'
    nulled but not freed upon failure [memleakOnRealloc]
  [src/rrd_cgi.c:1138] (error) Common realloc mistake: 'argv'
    nulled but not freed upon failure [memleakOnRealloc]
  [src/rrd_graph.c:1211] (error) Common realloc mistake: 'steparray'
    nulled but not freed upon failure [memleakOnRealloc]