bpo-30242: resolve some undefined behaviours in struct (#1418)

Modules/_struct.c

@@ -543,7 +543,7 @@ np_ubyte(char *p, PyObject *v, const formatdef *f)
                         "ubyte format requires 0 <= number <= 255");
         return -1;
     }
-    *p = (char)x;
+    *(unsigned char *)p = (unsigned char)x;
     return 0;
 }
 
@@ -864,6 +864,7 @@ bp_int(char *p, PyObject *v, const formatdef *f)
 {
     long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_long(v, &x) < 0)
         return -1;
     i = f->size;
@@ -876,7 +877,7 @@ bp_int(char *p, PyObject *v, const formatdef *f)
 #endif
     }
     do {
-        p[--i] = (char)x;
+        q[--i] = (unsigned char)(x & 0xffL);
         x >>= 8;
     } while (i > 0);
     return 0;
@@ -887,6 +888,7 @@ bp_uint(char *p, PyObject *v, const formatdef *f)
 {
     unsigned long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_ulong(v, &x) < 0)
         return -1;
     i = f->size;
@@ -897,7 +899,7 @@ bp_uint(char *p, PyObject *v, const formatdef *f)
             RANGE_ERROR(x, f, 1, maxint - 1);
     }
     do {
-        p[--i] = (char)x;
+        q[--i] = (unsigned char)(x & 0xffUL);
         x >>= 8;
     } while (i > 0);
     return 0;
@@ -1077,6 +1079,7 @@ lp_int(char *p, PyObject *v, const formatdef *f)
 {
     long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_long(v, &x) < 0)
         return -1;
     i = f->size;
@@ -1089,7 +1092,7 @@ lp_int(char *p, PyObject *v, const formatdef *f)
 #endif
     }
     do {
-        *p++ = (char)x;
+        *q++ = (unsigned char)(x & 0xffL);
         x >>= 8;
     } while (--i > 0);
     return 0;
@@ -1100,6 +1103,7 @@ lp_uint(char *p, PyObject *v, const formatdef *f)
 {
     unsigned long x;
     Py_ssize_t i;
+    unsigned char *q = (unsigned char *)p;
     if (get_ulong(v, &x) < 0)
         return -1;
     i = f->size;
@@ -1110,7 +1114,7 @@ lp_uint(char *p, PyObject *v, const formatdef *f)
             RANGE_ERROR(x, f, 1, maxint - 1);
     }
     do {
-        *p++ = (char)x;
+        *q++ = (unsigned char)(x & 0xffUL);
         x >>= 8;
     } while (--i > 0);
     return 0;