Custom transport fails when insufficient bytes read

[a_villacis]

This patch fixes a (IMHO) serious bug in the custom transport. If the custom transport read an insufficient (but still nonzero) number of bytes at a time, the read routine at lib/obex_main.c can enter a situation in which the first instance of /* Check if we are still connected */ consumes all the available bytes, and the second instance sees an empty buffer, which it (incorrectly) interprets as an EOF, thus triggering OBEX_EV_LINKERR. This bug prevents obex_test from correctly sending or receiving objects when using the custom (serial) transport when the custom transport fails to feed big enough chunks.

[a_villacis]

Check for buffer provided in custom transport scenario

[zany]

Applied as "do not error if the data is from non-empty but partial buffer (custom transport) ". Thanks!