awk: fix printf "%-10c", 0

function                                             old     new   delta
awk_printf                                           596     626     +30

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

testsuite/awk.tests

@@ -415,6 +415,14 @@ testing "awk printf('%c') can output NUL" \
 	"awk '{printf(\"hello%c null\n\", 0)}'" "hello\0 null\n" "" "\n"
 SKIP=
 
+optional FEATURE_AWK_GNU_EXTENSIONS
+testing "awk printf('%-10c') can output NUL" \
+	"awk 'BEGIN { printf \"[%-10c]\n\", 0 }' | od -tx1" "\
+0000000 5b 00 20 20 20 20 20 20 20 20 20 5d 0a
+0000015
+" "" ""
+SKIP=
+
 # testing "description" "command" "result" "infile" "stdin"
 testing 'awk negative field access' \
 	'awk 2>&1 -- '\''{ $(-1) }'\' \