Experimental fix for potential Negative array index read in gradiend slider
Coverity CID 1034359

(cherry picked from commit 5e57fafa0d78d74f89b3515c2250c3a77321bcba)
Fix potential out-of-bounds write
However, configstr can not be longer than ~700 chars (see src/control/conf.h, function dt_conf_init), so overflow is unlikely
Coverity CID 1034339

(cherry picked from commit f746cb79a721679d0920b1624c829cf4f7ac7d07)