civetweb.c: fix realloc handling

Fix handling of realloc. If realloc() fails it returns NULL, assigning
the return value of realloc() directly to the pointer without checking
for the result will lead to a memory leak in error case.

Use a temporary pointer to hold the result of realloc().

Fix for:
[src/civetweb.c:409]: (error) Common realloc mistake: 'data' nulled but
not freed upon failure

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>