C library/strdup: use calloc for an array that will be overwritten

calloc will zero-initialize the array, making it amenable to constant
propagation. If subsequent updates via strcpy write constants, we can keep
constant-propagating the array.

regression/cbmc-library/String6/strdup-calloc.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+--pointer-check --bounds-check --program-only
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+dynamic_object#\d+ WITH

src/ansi-c/library/string.c

@@ -571,7 +571,7 @@ inline char *strdup(const char *str)
   __CPROVER_HIDE:;
   __CPROVER_size_t bufsz;
   bufsz=(strlen(str)+1);
-  char *cpy=(char *)malloc(bufsz*sizeof(char));
+  char *cpy = (char *)calloc(bufsz * sizeof(char), sizeof(char));
   if(cpy==((void *)0)) return 0;
   #ifdef __CPROVER_STRING_ABSTRACTION
   __CPROVER_assume(__CPROVER_buffer_size(cpy)==bufsz);