Fail cleanly on allocation failures

Previously code in the SWIG tool didn't handle allocation failures
well.  Most places didn't check for NULL return from
malloc()/realloc()/calloc() at all, typically resulting in undefined
behaviour, and some places used assert() to check for a NULL return
(which is a misuse of assert() and such checks disappear if built with
NDEBUG defined leaving us back with undefined behaviour).

All C allocations are now done via wrapper functions (Malloc(),
Realloc() and Calloc()) which emit and error and exit with non-zero
status on failure, so a non-NULL return can be relied upon.

Fixes #1901.

Source/Modules/swigmain.cxx

@@ -98,15 +98,15 @@ static TargetLanguageModule modules[] = {
 void SWIG_merge_envopt(const char *env, int oargc, char *oargv[], int *nargc, char ***nargv) {
   if (!env) {
     *nargc = oargc;
-    *nargv = (char **)malloc(sizeof(char *) * (oargc + 1));
+    *nargv = (char **)Malloc(sizeof(char *) * (oargc + 1));
     memcpy(*nargv, oargv, sizeof(char *) * (oargc + 1));
     return;
   }
 
   int argc = 1;
   int arge = oargc + 1024;
-  char **argv = (char **) malloc(sizeof(char *) * (arge + 1));
-  char *buffer = (char *) malloc(2048);
+  char **argv = (char **) Malloc(sizeof(char *) * (arge + 1));
+  char *buffer = (char *) Malloc(2048);
   char *b = buffer;
   char *be = b + 1023;
   const char *c = env;
@@ -139,11 +139,11 @@ static void insert_option(int *argc, char ***argv, int index, char const *start,
   size_t option_len = end - start;
 
   // Preserve the NULL pointer at argv[argc]
-  new_argv = (char **)realloc(new_argv, (new_argc + 2) * sizeof(char *));
+  new_argv = (char **)Realloc(new_argv, (new_argc + 2) * sizeof(char *));
   memmove(&new_argv[index + 1], &new_argv[index], sizeof(char *) * (new_argc + 1 - index));
   new_argc++;
 
-  new_argv[index] = (char *)malloc(option_len + 1);
+  new_argv[index] = (char *)Malloc(option_len + 1);
   memcpy(new_argv[index], start, option_len);
   new_argv[index][option_len] = '\0';