Adds some missing error handling to PGTYPESnumeric_div() in ecpg's

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.