Fix nullptr dereference in utilities whan argv[0] == NULL

https://lwn.net/Articles/?offset=50 was an entertaining reading where we
learn that the fact that argv[0] contains the name of the binary is
purely a convention, normally taken by the shell that launches the
process, but not guaranteed by the execve() system call that does the
job.

The following test program tested against cct, cs2cs, geod, gie and proj
make them cause a null pointer dereference

```
 #include <unistd.h>
 #include <stdio.h>
extern char **environ;

int main()
{
    char* argv[] = { NULL };
    printf("%d\n", execve("/path/to/some/proj/binary", argv, environ));
    return 0;
}
```

1 files changed, 3 insertions, 0 deletions

diff --git a/src/apps/optargpm.h b/src/apps/optargpm.h
index 9a66b9a0..921b8c0b 100644
--- a/src/apps/optargpm.h
+++ b/src/apps/optargpm.h
@@ -415,6 +415,9 @@ OPTARGS *opt_parse (int argc, char **argv, const char *flags, const char *keys,
     int free_format;
     OPTARGS *o;
 
+    if( argc == 0 )
+        return nullptr;
+
     o = (OPTARGS *) calloc (1, sizeof(OPTARGS));
     if (nullptr==o)
         return nullptr;