lib/utils_base.c: if asprintf fails, string is undefined

if asprintf fails, string content becomes invalid. we need
to check if it ran OK by checking the returned value.

in case of fail, asprintf returns -1, otherwise the number
of writen bytes is returned.

also, on ubuntu 13.10 i've receiving a lot of warnings:

"warning: ignoring return value of ‘asprintf’"

this patches fixes some of them

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>
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Closes #1227
Make use of xasprintf instead of asprintf
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Closes #1278
Merge pull request #1278 from waja/github1227