Revert ECPG's use of pnstrdup()

Commit 0b9466fce added a dependency on fe_memutils' pnstrdup() inside
informix.c.  This adds an exit() path in a library, which we don't
want.  (Unlike libpq, the ecpg libraries don't have an automated check
for that, but it makes sense to keep them to a similar standard.)  The
ecpg code can already handle failure results from the *strdup() call
by itself.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/CAOYmi+=pg=W5L1h=3MEP_EB24jaBu2FyATrLXqQHGe7cpuvwyg@mail.gmail.com

src/interfaces/ecpg/compatlib/informix.c

@@ -175,6 +175,25 @@ deccopy(decimal *src, decimal *target)
 	memcpy(target, src, sizeof(decimal));
 }
 
+static char *
+ecpg_strndup(const char *str, size_t len)
+{
+	size_t		real_len = strlen(str);
+	int			use_len = (int) ((real_len > len) ? len : real_len);
+
+	char	   *new = malloc(use_len + 1);
+
+	if (new)
+	{
+		memcpy(new, str, use_len);
+		new[use_len] = '\0';
+	}
+	else
+		errno = ENOMEM;
+
+	return new;
+}
+
 int
 deccvasc(const char *cp, int len, decimal *np)
 {
@@ -186,8 +205,8 @@ deccvasc(const char *cp, int len, decimal *np)
 	if (risnull(CSTRINGTYPE, cp))
 		return 0;
 
-	str = pnstrdup(cp, len);	/* decimal_in always converts the complete
+	str = ecpg_strndup(cp, len);	/* decimal_in always converts the complete
-								 * string */
+									 * string */
 	if (!str)
 		ret = ECPG_INFORMIX_NUM_UNDERFLOW;
 	else