Fix a problem in FcConfigSubstitute

We were using the family names from the pattern without
copying, and this was leading to a valgrind warning:
==53167== Invalid read of size 1
==53167==    at 0x58B0238: FcStrCaseWalkerNextNonBlank (fcstr.c:198)
==53167==    by 0x58B0238: FcStrCaseWalkerNextNonBlank (fcstr.c:186)
==53167==    by 0x58B02C7: FcStrCmpIgnoreBlanksAndCase (fcstr.c:281)
==53167==    by 0x58A4D44: FcHashTableFind (fchash.c:109)
==53167==    by 0x5895E76: FamilyTableAdd (fccfg.c:1634)
==53167==    by 0x589646A: FcConfigAdd.isra.0 (fccfg.c:1823)
==53167==    by 0x58988CF: IA__FcConfigSubstituteWithPat.part.0 (fccfg.c:2228)
==53167==    by 0x55F4F1A: pango_cairo_fc_font_map_fontset_key_substitute (pangocairo-fcfontmap.c:106)
==53167==    by 0x5B88AF6: pango_fc_default_substitute (pangofc-fontmap.c:1795)
==53167==    by 0x5B88D15: pango_fc_font_map_get_patterns (pangofc-fontmap.c:1850)
==53167==    by 0x5B88FC7: pango_fc_font_map_load_fontset (pangofc-fontmap.c:1952)
==53167==    by 0x5623627: pango_font_map_load_fontset (pango-fontmap.c:161)
==53167==    by 0x5621743: pango_context_get_metrics (pango-context.c:1782)
==53167==  Address 0x150d3450 is 0 bytes inside a block of size 10 free'd
==53167==    at 0x483B9F5: free (vg_replace_malloc.c:538)
==53167==    by 0x58ABE70: FcValueListDestroy (fcpat.c:147)
==53167==    by 0x5898A08: IA__FcConfigSubstituteWithPat.part.0 (fccfg.c:2203)
==53167==    by 0x55F4F1A: pango_cairo_fc_font_map_fontset_key_substitute (pangocairo-fcfontmap.c:106)
==53167==    by 0x5B88AF6: pango_fc_default_substitute (pangofc-fontmap.c:1795)
==53167==    by 0x5B88D15: pango_fc_font_map_get_patterns (pangofc-fontmap.c:1850)
==53167==    by 0x5B88FC7: pango_fc_font_map_load_fontset (pangofc-fontmap.c:1952)
==53167==    by 0x5623627: pango_font_map_load_fontset (pango-fontmap.c:161)
==53167==    by 0x5621743: pango_context_get_metrics (pango-context.c:1782)

Use copies of the strings as keys in the hash table to avoid this.
10 jobs for fccfg-bad-access in 6 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #4323377
gst-macos-10.15
meson macos

00:00:49

passed #4323374
meson-mingw-w64-build

00:04:50

manual #4323378
docker windows 1809 allowed to fail manual
meson msys2
passed #4323372
meson-shared-build

00:04:04

passed #4323373
meson-static-build

00:03:28

passed #4323375
docker windows 1809
meson vs2017 amd64

00:01:08

passed #4323376
docker windows 1809
meson vs2017 x86

00:01:08

passed #4323371
mingw-build

00:06:06

passed #4323369
shared-build

00:05:58

passed #4323370
static-build

00:06:26