makecpt -G on CPTs with custon label info fails

[PaulWessel]

Description of the problem

When trying to limit the remote CTP @age_chrons_GTS2012_2020.cpt on z:

   0.0  219/30/38        11.06  219/30/38       U       ;5o
 11.06  241/71/28        19.72  241/71/28       U       ;6o
 19.72  245/114/21       33.16  245/114/21      U       ;13y
....

I get lots of memory issues and the result has lost all the custom label information:

gmt makecpt -C@age_chrons_GTS2012_2020.cpt -G0/164.70 > t.cpt
makecpt (gmt_support.c:1138(gmtsupport_free_range)): Wrongly tries to free item
makecpt (gmt_support.c:1138(gmtsupport_free_range)): Wrongly tries to free item
makecpt (gmt_support.c:1138(gmtsupport_free_range)): Wrongly tries to free item
....

head -3 t.cpt

0	219/30/38	11.06	219/30/38
11.06	241/71/28	19.72	241/71/28
19.72	245/114/21	33.16	245/114/21

Yet, when used by colorbar there is no issues:

gmt colorbar -C@age_chrons_GTS2012_2020.cpt -G0/164.70 -Dn0.015/0.055+jBL+w5.8i/0.12c+h -S+c --FONT_ANNOT_PRIMARY=3.5p -Rd -JQ180/6i -pdf map

Will require a check and perhaps debug to see why the different behaviors.