Print: stop leaking dests allocated by cupsGetDests()

Fixes several LeakSanitizer-detected leaks like:
Direct leak of 32 byte(s) in 1 object(s) allocated from:
    #0 0x7ff195fd474f in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x5474f)
    #1 0x7ff18e98282d (/usr/lib/x86_64-linux-gnu/libcups.so.2+0x2282d)
    #2 0x7ff18e9845a5 in _cupsGetDests (/usr/lib/x86_64-linux-gnu/libcups.so.2+0x245a5)
    #3 0x7ff18e98553b in cupsGetDests2 (/usr/lib/x86_64-linux-gnu/libcups.so.2+0x2553b)
    #4 0x7ff195da8c8d in dt_get_papers /home/lebedevri/darktable/src/common/cups_print.c:189
    #5 0x7ff16987610e
    #6 0x7ff169879cc2
    #7 0x7ff195d42cb7 in dt_lib_load_modules /home/lebedevri/darktable/src/libs/lib.c:718
    #8 0x7ff195d45586 in dt_lib_init /home/lebedevri/darktable/src/libs/lib.c:1008
    #9 0x7ff195b3e8c7 in dt_init /home/lebedevri/darktable/src/common/darktable.c:889
    #10 0x400cb8 in main /home/lebedevri/darktable/src/main.c:24
    #11 0x7ff18e3a9b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    #12 0x400bd8 (/usr/local/bin/darktable+0x400bd8)

Indirect leak of 304 byte(s) in 1 object(s) allocated from:
    #0 0x7ff195fd4a06 in __interceptor_realloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x54a06)
    #1 0x7ff18e9a4a58 in cupsAddOption (/usr/lib/x86_64-linux-gnu/libcups.so.2+0x44a58)
    #2 0x7ff18e9847f2 in _cupsGetDests (/usr/lib/x86_64-linux-gnu/libcups.so.2+0x247f2)
    #3 0x7ff18e98553b in cupsGetDests2 (/usr/lib/x86_64-linux-gnu/libcups.so.2+0x2553b)
    #4 0x7ff195da8c8d in dt_get_papers /home/lebedevri/darktable/src/common/cups_print.c:189
    #5 0x7ff16987610e
    #6 0x7ff169879cc2
    #7 0x7ff195d42cb7 in dt_lib_load_modules /home/lebedevri/darktable/src/libs/lib.c:718
    #8 0x7ff195d45586 in dt_lib_init /home/lebedevri/darktable/src/libs/lib.c:1008
    #9 0x7ff195b3e8c7 in dt_init /home/lebedevri/darktable/src/common/darktable.c:889
    #10 0x400cb8 in main /home/lebedevri/darktable/src/main.c:24
    #11 0x7ff18e3a9b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    #12 0x400bd8 (/usr/local/bin/darktable+0x400bd8)

Author: LebedevRI

src/common/cups_print.c

@@ -231,6 +231,8 @@ GList *dt_get_papers(const char *printer_name)
     else
       dt_print(DT_DEBUG_PRINT, "[print] cannot connect to printer %s (cancel=%d)\n", printer_name, cancel);
   }
+
+  cupsFreeDests(num_dests, dests);
 #endif
 
   // check now PPD page sizes
@@ -288,6 +290,8 @@ void dt_print_file(const int32_t imgid, const char *filename, const dt_print_inf
                                   dest->options[j].value,
                                   num_options, &options);
 
+  cupsFreeDests(num_dests, dests);
+
   // disable cm on CUPS, this is important as dt does the cm
 
   if (*pinfo->printer.profile)