Add checks for valid multibyte character length in UtfToLocal, LocalToUtf.

This is mainly to suppress "uninitialized variable" warnings from very
recent versions of gcc.  But it seems like a good robustness thing anyway,
not to mention that we might someday decide to support 6-byte UTF8.

Per report from Karol Trzcionka.  No back-patch since there's no reason
at the moment to think this is more than cosmetic.

src/backend/utils/mb/conv.c

@@ -382,6 +382,11 @@ UtfToLocal(const unsigned char *utf, unsigned char *iso,
 			iutf |= *utf++ << 8;
 			iutf |= *utf++;
 		}
+		else
+		{
+			elog(ERROR, "unsupported character length %d", l);
+			iutf = 0;			/* keep compiler quiet */
+		}
 
 		/*
 		 * first, try with combined map if possible
@@ -437,6 +442,11 @@ UtfToLocal(const unsigned char *utf, unsigned char *iso,
 				iutf |= *utf++ << 8;
 				iutf |= *utf++;
 			}
+			else
+			{
+				elog(ERROR, "unsupported character length %d", l);
+				iutf = 0;		/* keep compiler quiet */
+			}
 
 			cutf[1] = iutf;
 			cp = bsearch(cutf, cmap, size2,
@@ -546,6 +556,11 @@ LocalToUtf(const unsigned char *iso, unsigned char *utf,
 			iiso |= *iso++ << 8;
 			iiso |= *iso++;
 		}
+		else
+		{
+			elog(ERROR, "unsupported character length %d", l);
+			iiso = 0;			/* keep compiler quiet */
+		}
 
 		p = bsearch(&iiso, map, size1,
 					sizeof(pg_local_to_utf), compare2);