pgsql: Adds some missing error handling to PGTYPESnumeric_div() in

Log Message:
-----------
Adds some missing error handling to PGTYPESnumeric_div() in ecpg's
pgtypeslib: (1) we need to check the return value of sub_abs() (2) we
need to check the return value of 4 calls to digitbuf_alloc().

Per Coverity static analysis performed by EnterpriseDB.

Modified Files:
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pgsql/src/interfaces/ecpg/pgtypeslib:
numeric.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/numeric.c.diff?r1=1.21&r2=1.22)