pmap: ensure arg1 is not null, and set qualifers to match with usage [smatch scan]

pmap: ensure arg1 is not null, and set qualifers to match with usage [smatch scan]

pmap.c:436 main(59) error: we previously assumed 'arg1' could be null (see line 427)
pmap.c:136:3: warning: return discards 'const' qualifier from pointer target type [enabled by default]
pmap.c:142:4: warning: return discards 'const' qualifier from pointer target type [enabled by default]
pmap.c:150:2: warning: return discards 'const' qualifier from pointer target type [enabled by default]
pmap.c:300:8: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>