Don't return values when doing inverse projections outside of the mollweide map

[mraspaud]

When doing invproj outside of the map with +proj=mollweide, valid lon/lat values were returned:
eg:

~$ invproj +proj=moll +a=6371000
-9000000 -9000000
113d0'27.032"W    89d27'43.361"S
^C
~$ proj +proj=moll +a=6371000
113d0'27.032"W    89d27'43.361"S
-0.00    -9009954.61

The effect of this can be seen here:

This patch fixes the behaviour by returning HUGE_VAL in the invproj call when outside the map.

[mraspaud]

Here is how it looks now:

~$ invproj +proj=moll +a=6371000
-9000000 -9000000
-2147483648d-2147483648'-nan"E  -2147483648d-2147483648'-nan"N