i.group segfaults on empty input

[neteler]

When running i.group with *empty* input (as it can happen from an unsuccessful g.mlist call piped into it), it segfaults:

i.group group=tmean_avg input=""
Segmentation fault (core dumped)

The real call was:

i.group group=tmean_avg input=`g.mlist rast pat="tmean.1981_2010.??.avg sep=,"`
Segmentation fault (core dumped)

but g.mlist did not return anything due to a typo in the map name pattern.

GDB (line broken for readability):

(gdb) run group=tmean_0_25deg_1981_2010_avg \
  input=`g.mlist rast pat="tmean_0_25deg.1981_2010.??.avg sep=,"`

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7bb2ee6 in G_name_is_fully_qualified \
(fullname=0x0, name=0x7fffffffd410 "", mapset=0x7fffffffd310 "") 
at nme_in_mps.c:45

[martinl]

In 69227:

i.group segfaults on empty input (see #1705)

[martinl]

In 69228:

i.group segfaults on empty input (see #1705) (merge r69227 from trunk)

[martinl]

In 69229:

i.group segfaults on empty input (fix #1705) (merge r69227 from trunk)