Clean up and ensure proper usage of alloca()

This replaces all calls to alloca() with ast_alloca() which calls gcc's
__builtin_alloca() to avoid BSD semantics and removes all NULL checks
on memory allocated via ast_alloca() and ast_strdupa().

(closes issue ASTERISK-20125)
Review: https://reviewboard.asterisk.org/r/2032/
Patch-by: Walter Doekes (wdoekes)
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Merged revisions 370642 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 370643 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370655 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/utils.h

@@ -646,6 +646,15 @@ int _ast_vasprintf(char **ret, const char *file, int lineno, const char *func, c
 
 #endif /* AST_DEBUG_MALLOC */
 
+/*!
+  \brief call __builtin_alloca to ensure we get gcc builtin semantics
+  \param size The size of the buffer we want allocated
+
+  This macro will attempt to allocate memory from the stack.  If it fails
+  you won't get a NULL returned, but a SEGFAULT if you're lucky.
+*/
+#define ast_alloca(size) __builtin_alloca(size)
+
 #if !defined(ast_strdupa) && defined(__GNUC__)
 /*!
  * \brief duplicate a string in memory from the stack