Check calloc return value in csrc/addon.cc to avoid NULL pointer dereference

[grgalex]

We don't check the return value of calloc [1], which can be NULL if the allocation fails.

This can lead to a Segmentation Fault when we later try to write to the result buffer [2].

[1]

ttf2woff2/csrc/addon.cc

Line 27 in ceea281

char* output_data = (char*) calloc(max_output_length, 1);

[2]

ttf2woff2/csrc/woff2/woff2_enc.cc

Line 355 in ceea281

StoreU32(kWoff2Signature, &offset, result);