UMM_CALLOC doesn't check for NULL allocation

[earlephilhower]

Found in debug of #4134

Arduino/cores/esp8266/umm_malloc/umm_malloc.c

Line 1688 in 0fe7259

memset(ret, 0x00, size);

UMM_CALLOC does an alloc then memset(0), but doesn't check the malloc succeeded, leading to a crash in UMM.

[earlephilhower]

Closing, this was merged.