reinstate removal of mon guard from is_safemon()

callers were checked:
domove_attackmon_at(mtmp, x, y, displaceu) has mtmp declared nonnull;
    there are dereferences of mtmp in the first line of code in
    the function.

In domove_core():
    The 1st occurrence of is_safemon(mtmp) is guarded by if (mtmp) { }.
    The 2nd occurrence of is_safemon(mtmp) is inside an if (mtmp) { } block.
    The 3rd occurrence of is_safemon(mtmp) was just remediated by 987be7e8.

In lookaround():
    The only occurrence of is_safemon(mtmp) is inside an
        if ((mtmp = m_at(x, y)) != 0 [...] { } block.

In do_attack(mtmp), in uhitm.c:
    The parameter is declared NONNULLARG1, and the 1st line of
    code contains a dereference with mtmp->data, which would
    segfault if mtmp were NULL.

include/display.h

@@ -157,7 +157,7 @@
  * definition here is convenient.  No longer limited to pets.
  */
 #define _is_safemon(mon) \
-    (flags.safe_dog && (mon) && (mon)->mpeaceful && canspotmon(mon)     \
+    (flags.safe_dog && (mon)->mpeaceful && canspotmon(mon)              \
      && !Confusion && !Hallucination && !Stunned)
 
 /*

src/hack.c

@@ -1768,6 +1768,7 @@ domove_attackmon_at(
     coordxy x, coordxy y,
     boolean *displaceu)
 {
+    /* assert(mtmp != NULL) */
     /* only attack if we know it's there
      * or if we used the 'F' command to fight blindly
      * or if it hides_under, in which case we call do_attack() to print