Add xmalloc and xrealloc wrappers

Add xmalloc and xrealloc wrappers

If malloc  or realloc fail they return NULL. Theorically this condition
should be tested in the code, but it's a strange condition today (basically
if this is hapenning thenyou have a big problem), and even Linux never returns
NULL in the default configuration (only if the process don't have room in
the space address, something a bit impossible in the case of st). But stis
enough small for being executed in low resources computers where this can be
a real problem. So the easy way is creating a wrappers function for them and
call to die in case of error.
---
 st.c |   44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)