var: use a dedicated pointer for the alloc

As the 'c' pointer might actually get modified before it is time to free
the memory.

Verify in test 2310

Reported-by: Eunsoo Kim
Fixes #21898
Closes #21900

tests/data/Makefile.am

@@ -258,7 +258,7 @@ test2100 test2101 test2102 test2103 test2104 test2105 \
 test2200 test2201 test2202 test2203 test2204 test2205 test2206 test2207 \
 \
 test2300 test2301 test2302 test2303 test2304 test2306 test2307 test2308 \
-test2309 \
+test2309 test2310 \
 \
 test2400 test2401 test2402 test2403 test2404 test2405 test2406 test2407 \
 test2408 test2409 test2410 test2411 \

tests/data/test2310

@@ -0,0 +1,50 @@
+<?xml version="1.0" encoding="US-ASCII"?>
+<testcase>
+<info>
+<keywords>
+variables
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data crlf="headers">
+HTTP/1.1 200 OK
+Date: Tue, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+ETag: "21025-dc7-39462498"
+Accept-Ranges: bytes
+Content-Length: 6
+Connection: something-close, close-something, close
+Content-Type: text/html
+Funny-head: yesyes
+
+-foo-
+</data>
+</reply>
+
+# Client-side
+<client>
+<server>
+http
+</server>
+<name>
+variable decode and trim
+</name>
+<command>
+--variable 'VAR=IA==' --expand-url 'http://%HOSTIP:%HTTPPORT/{{VAR:64dec:trim}}'
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<protocol crlf="headers">
+GET / HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+User-Agent: curl/%VERSION
+Accept: */*
+
+</protocol>
+</verify>
+</testcase>