Fixed error handling in numeric conversion (Joachim).

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/include/pgtypes_numeric.h

src/interfaces/ecpg/pgtypeslib/numeric.c

src/interfaces/ecpg/test/expected/pgtypeslib-num_test2.c

...interfaces/ecpg/test/expected/pgtypeslib-num_test2.stdout

src/interfaces/ecpg/test/pgtypeslib/num_test2.pgc