python - be: Use snprintf instead of asprintf.

Log Message:
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Use snprintf instead of asprintf.

snprintf is likely to be more portable than asprintf, and the string is actually
simple enough that calculating the resulting size is trivial. Also, it can then
print right to the palloc'd memory instead of pstrdup'ing, saving a malloc/free.

Modified Files:
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    be/src:
        pl.c (r1.33 -> r1.34)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/pl.c.diff?r1=1.33&r2=1.34)