The error() function may return under exceptional circumstances (#97)

There are a few places where error() was called that in practice would never
be returned from, however since it was coded with the possibility of returning
all call sites must be consistent with that.

	modified:   ../src/car-cdr.c
	modified:   ../src/gchtfind.c
	modified:   ../src/hardrtn.c

src/car-cdr.c

@@ -78,6 +78,7 @@ LispPTR car(register LispPTR datum)
       return (NIL);
     else
       error("car : ARG not list");
+    return (NIL); /* NOT REACHED */
   }
 } /* end of car */
 
@@ -150,6 +151,7 @@ LispPTR rplaca(register LispPTR x, register LispPTR y)
         return (NIL_PTR);
     } else
       error("ARG not List");
+      return (NIL_PTR); /* NOT REACHED */
   }
 
   else {

src/gchtfind.c

@@ -94,6 +94,7 @@
         IncAllocCnt(1);                                               \
         return ptr; /* new 0 entry */                                 \
       default: error("GC error: new entry touches stack bit");        \
+        return NIL; /* NOT REACHED */                                 \
     }                                                                 \
   }
 
@@ -114,6 +115,7 @@
         GETGC(entry) = hiptr | (1 << HTCNTSHIFT) | HTSTKMASK;           \
         return NIL;                                                     \
       default: error("GC error: new entry when turning off stack bit"); \
+        return NIL; /* NOT REACHED */                                   \
     }                                                                   \
   }
 

src/hardrtn.c

@@ -250,7 +250,8 @@ retry: /* this is retry entry after MAKE_FXCOPY etc */
     else
       error("Shouldn't");
   }
-
+  error("Control reached end of slowreturn()!");
+  return (NIL); /* NOT REACHED */
 } /* slowreturn end */
 
 #define MAXSAFEUSECOUNT 200