awk: FS regex matches only non-empty separators (gawk compat)

function                                             old     new   delta
awk_split                                            484     553     +69

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -398,5 +398,12 @@ testing 'awk do not allow "str"++' \
 	'' \
 	'anything'
 
+#gawk compat: FS regex matches only non-empty separators:
+#with -*, the splitting is NOT f o o b a r, but foo bar:
+testing 'awk FS regex which can match empty string' \
+	"awk -F '-*' '{print \$1 \"-\" \$2 \"=\" \$3 \"*\" \$4}'" \
+	"foo-bar=*\n" \
+	'' \
+	'foo--bar'
 
 exit $FAILCOUNT