Avoid infinite loop if command "fpm-" is in path (#713)

If a copy of fpm is renamed fpm- it will be called recursively
because plugin-names are searched for in the form of "fpm-*"
because since there is no test if the plugin suffix is blank
the "fpm-" command will be called.

A relatively minor bug; but it takes a one-line change to fix
and is quite confusing to anyone with a "fpm-" command in their
path.

Author: urbanjost

src/fpm_command_line.f90

@@ -574,7 +574,8 @@ subroutine get_command_line_settings(cmd_settings)
 
         case default
 
-            if(which('fpm-'//cmdarg).ne.'')then
+            if(cmdarg.ne.''.and.which('fpm-'//cmdarg).ne.'')then
+
                 call run('fpm-'//trim(cmdarg)//' '// get_command_arguments_quoted(),.false.)
             else
                 call set_args('&