fix -IV_MIN negations

Doing uv = -iv is undefined behaviour if iv happens to be IV_MIN.
This occurs in several places in the perl sources.

Found by -fsanitize=undefined.

Here's a typical message:

sv.c:2864:7: runtime error: negation of -9223372036854775808 cannot be represented in type 'IV' (aka 'long'); cast to an unsigned type to negate this value to itself

1 files changed, 5 insertions, 4 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 3ee48180d3..4072ab11e1 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -647,8 +647,8 @@ PP(pp_add)
 		    if (aiv >= 0) {
 			auv = aiv;
 			auvok = 1;	/* Now acting as a sign flag.  */
-		    } else { /* 2s complement assumption for IV_MIN */
-			auv = (UV)-aiv;
+		    } else {
+			auv = (aiv == IV_MIN) ? (UV)aiv : (UV)(-aiv);
 		    }
 		}
 		a_valid = 1;
@@ -668,7 +668,7 @@ PP(pp_add)
 		    buv = biv;
 		    buvok = 1;
 		} else
-		    buv = (UV)-biv;
+                    buv = (biv == IV_MIN) ? (UV)biv : (UV)(-biv);
 	    }
 	    /* ?uvok if value is >= 0. basically, flagged as UV if it's +ve,
 	       else "IV" now, independent of how it came in.
@@ -709,7 +709,8 @@ PP(pp_add)
 		else {
 		    /* Negate result */
 		    if (result <= (UV)IV_MIN)
-			SETi( -(IV)result );
+                        SETi(result == (UV)IV_MIN
+                                ? IV_MIN : -(IV)result);
 		    else {
 			/* result valid, but out of range for IV.  */
 			SETn( -(NV)result );