awk: Guard pointer chasing when parsing ternary expressions.

Avoids an uninit pointer deref for some malformed ternary exprs.

Add a test that would crash in busybox before this fix.

Signed-off-by: Brian Foley <bpfoley@google.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -338,6 +338,9 @@ testing "awk continue" \
 testing "awk handles invalid for loop" \
     "awk '{ for() }' 2>&1" "awk: cmd. line:1: Unexpected token\n" "" ""
 
+testing "awk handles colon not preceded by ternary" \
+    "awk 'foo:bar:' 2>&1" "awk: cmd. line:1: Unexpected token\n" "" ""
+
 # testing "description" "command" "result" "infile" "stdin"
 testing 'awk negative field access' \
 	'awk 2>&1 -- '\''{ $(-1) }'\' \