mprintf: allow %.s with data not being zero terminated

If the precision is indeed shorter than the string, don't strlen() to
find the end because that's not how the precision operator works.

I also added a unit test for curl_msnprintf to make sure this works and
that the fix doesn't a few other basic use cases. I found a POSIX
compliance problem that I marked TODO in the unit test, and I figure we
need to add more tests in the future.

Reported-by: Török Edwin

tests/data/test1398

@@ -0,0 +1,26 @@
+<testcase>
+<info>
+<keywords>
+unittest
+curl_msnprintf
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+</features>
+ <name>
+curl_msnprintf unit tests
+ </name>
+<tool>
+unit1398
+</tool>
+</client>
+
+</testcase>