ICU-20544 Regex, fix min/max match length computation with negative look-behind patterns.

icu4c/source/i18n/regexcmp.cpp

diff --git a/icu4c/source/i18n/regexcmp.cpp b/icu4c/source/i18n/regexcmp.cpp
index 2d14aa8..dcd754d 100644
--- a/icu4c/source/i18n/regexcmp.cpp
+++ b/icu4c/source/i18n/regexcmp.cpp
@@ -2328,7 +2328,7 @@ void  RegexCompile::handleCloseParen() {
                 error(U_REGEX_LOOK_BEHIND_LIMIT);
                 break;
             }
-            if (minML == INT32_MAX && maxML == 0) {
+            if (minML == INT32_MAX) {
                 // This condition happens when no match is possible, such as with a
                 // [set] expression containing no elements.
                 // In principle, the generated code to evaluate the expression could be deleted,

icu4c/source/test/testdata/regextst.txt

diff --git a/icu4c/source/test/testdata/regextst.txt b/icu4c/source/test/testdata/regextst.txt
index ba84af0..552f067 100644
--- a/icu4c/source/test/testdata/regextst.txt
+++ b/icu4c/source/test/testdata/regextst.txt
@@ -1439,13 +1439,16 @@
 "[^\u0000-\U0010ffff]"             "a"
 "[^[^\u0000-\U0010ffff]]"          "<0>a</0>"
 
+# Bug ICU-20544. Similar to 20385, above. Assertion failure with a negative look-behind assertion containing
+#                a set with no contents. Look-behind pattern includes more than just the empty set.
+
+"(?<![ⰿ&&m]c)"                     "<0></0>abc"   # note: first 'ⰿ' is \u2c3f, hence empty set.
+"(?<![^\u0000-\U0010ffff]c)"       "<0></0>abc"
 
 #  Random debugging, Temporary
 #
 
 
-
-
 #
 #  Regexps from http://www.regexlib.com
 #