ext/standard/config.m4: fix crypt() test segfaults in >=glibc-2.17.

Starting with glibc-2.17, the crypt() function will report an EINVAL
and return NULL when the format of the "salt" parameter is
invalid. The current tests for crypt() pass its result to strcmp(),
causing segfaults when the value returned from crypt() is NULL.

This commit modifies the test programs to exit with failure when
crypt() returns NULL.

Reference: https://bugs.gentoo.org/show_bug.cgi?id=518964

Author: orlitzky

ext/standard/config.m4

@@ -71,7 +71,8 @@ AC_CACHE_CHECK(for standard DES crypt, ac_cv_crypt_des,[
 
 main() {
 #if HAVE_CRYPT
-    exit (strcmp((char *)crypt("rasmuslerdorf","rl"),"rl.3StKT.4T8M"));
+    char* encrypted = crypt("rasmuslerdorf","rl");
+    exit (!encrypted || strcmp(encrypted,"rl.3StKT.4T8M"));
 #else
 	exit(0);
 #endif
@@ -95,7 +96,8 @@ AC_CACHE_CHECK(for extended DES crypt, ac_cv_crypt_ext_des,[
 
 main() {
 #if HAVE_CRYPT
-  exit (strcmp((char *)crypt("rasmuslerdorf","_J9..rasm"),"_J9..rasmBYk8r9AiWNc"));
+  char* encrypted = crypt("rasmuslerdorf","_J9..rasm");
+  exit (!encrypted || strcmp(encrypted,"_J9..rasmBYk8r9AiWNc"));
 #else
   exit(0);
 #endif
@@ -128,7 +130,8 @@ main() {
     salt[12]='\0';
     strcpy(answer,salt);
     strcat(answer,"rISCgZzpwk3UhDidwXvin0");
-    exit (strcmp((char *)crypt("rasmuslerdorf",salt),answer));
+    char* encrypted = crypt("rasmuslerdorf",salt);
+    exit (!encrypted || strcmp(encrypted,answer));
 #else
 	exit(0);
 #endif
@@ -158,7 +161,8 @@ main() {
     strcat(salt,"rasmuslerd............");
     strcpy(answer,salt);
     strcpy(&answer[29],"nIdrcHdxcUxWomQX9j6kvERCFjTg7Ra");
-    exit (strcmp((char *)crypt("rasmuslerdorf",salt),answer));
+    char* encrypted = crypt("rasmuslerdorf",salt);
+    exit (!encrypted || strcmp(encrypted,answer));
 #else
 	exit(0);
 #endif
@@ -187,7 +191,8 @@ main() {
     strcpy(salt,"\$6\$rasmuslerdorf\$");
     strcpy(answer, salt);
     strcat(answer, "EeHCRjm0bljalWuALHSTs1NB9ipEiLEXLhYeXdOpx22gmlmVejnVXFhd84cEKbYxCo.XuUTrW.RLraeEnsvWs/");
-    exit (strcmp((char *)crypt("rasmuslerdorf",salt),answer));
+    char* encrypted = crypt("rasmuslerdorf",salt);
+    exit (!encrypted || strcmp(encrypted,answer));
 #else
 	exit(0);
 #endif
@@ -216,7 +221,8 @@ main() {
     strcpy(salt,"\$5\$rasmuslerdorf\$");
     strcpy(answer, salt);
     strcat(answer, "cFAm2puLCujQ9t.0CxiFIIvFi4JyQx5UncCt/xRIX23");
-    exit (strcmp((char *)crypt("rasmuslerdorf",salt),answer));
+    char* encrypted = crypt("rasmuslerdorf",salt);
+    exit (!encrypted || strcmp(encrypted,answer));
 
 #else
 	exit(0);