Remove three unneeded variable assignments.

Found using Clang's static analyzer.

Python/ast.c

@@ -3378,17 +3378,12 @@ decode_utf8(struct compiling *c, const char **sPtr, const char *end, char* encod
 static PyObject *
 decode_unicode(struct compiling *c, const char *s, size_t len, int rawmode, const char *encoding)
 {
-        PyObject *v, *u;
+        PyObject *v;
+        PyObject *u = NULL;
         char *buf;
         char *p;
         const char *end;
-        if (encoding == NULL) {
-                buf = (char *)s;
-                u = NULL;
-        } else if (strcmp(encoding, "iso-8859-1") == 0) {
-                buf = (char *)s;
-                u = NULL;
-        } else {
+        if (encoding != NULL && strcmp(encoding, "iso-8859-1")) {
                 /* check for integer overflow */
                 if (len > PY_SIZE_MAX / 6)
                         return NULL;

Python/ceval.c

@@ -2697,7 +2697,6 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 			    Py_DECREF(*pfunc);
 			    *pfunc = self;
 			    na++;
-			    n++;
 		    } else
 			    Py_INCREF(func);
 		    sp = stack_pointer;

Python/dtoa.c

@@ -1382,7 +1382,6 @@ bigcomp(U *rv, const char *s0, BCinfo *bc)
     Bigint *b, *d;
     int b2, d2, dd, i, nd, nd0, odd, p2, p5;
 
-    dd = 0; /* silence compiler warning about possibly unused variable */
     nd = bc->nd;
     nd0 = bc->nd0;
     p5 = nd + bc->e0;
@@ -2362,7 +2361,7 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
 
     /* set pointers to NULL, to silence gcc compiler warnings and make
        cleanup easier on error */
-    mlo = mhi = b = S = 0;
+    mlo = mhi = S = 0;
     s0 = 0;
 
     u.d = dd;
@@ -2713,8 +2712,6 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
      * and for all and pass them and a shift to quorem, so it
      * can do shifts and ors to compute the numerator for q.
      */
-    if ((i = ((s5 ? 32 - hi0bits(S->x[S->wds-1]) : 1) + s2) & 0x1f))
-        i = 32 - i;
 #define iInc 28
     i = dshift(S, s2);
     b2 += i;

Python/getcwd.c

@@ -59,14 +59,13 @@ getcwd(char *buf, int size)
 {
 	FILE *fp;
 	char *p;
-	int sts;
 	if (size <= 0) {
 		errno = EINVAL;
 		return NULL;
 	}
 	if ((fp = popen(PWD_CMD, "r")) == NULL)
 		return NULL;
-	if (fgets(buf, size, fp) == NULL || (sts = pclose(fp)) != 0) {
+	if (fgets(buf, size, fp) == NULL || pclose(fp) != 0) {
 		errno = EACCES; /* Most likely error */
 		return NULL;
 	}