S_find_locale_from_environment: Handle disparate LC_ALL

Prior to this commit, it was possible on some platforms to improperly
set a locale category from a locale of "".

Consider:

export LC_ALL="LC_CTYPE=de_DE;LC_NUMERIC=C;LC_TIME=de_DE;..."

and then run

    print POSIX::setlocale(LC_NUMERIC, "");

It wouldn't know how to extract just the LC_NUMERIC part of the LC_ALL
string.

There is no test added because I didn't find a shell that allows setting
LC_ALL to such a disparate value.

locale.c

@@ -3123,7 +3123,42 @@ S_find_locale_from_environment(pTHX_ const locale_category_index index)
     /* Use any "LC_ALL" environment variable, as it overrides everything else.
      * */
     if (lc_all && strNE(lc_all, "")) {
-        return lc_all;
+        if (index == LC_ALL_INDEX_) {
+            return lc_all;
+        }
+
+        /* There is an LC_ALL environment variable, but we want only one
+         * component of it.  Split the result into its individual components */
+        switch (parse_LC_ALL_string(lc_all,
+                                    (const char **) &locale_names,
+                                    no_override,  /* Handled by other code */
+                                    false,    /* Return only [0] if suffices */
+                                    false,    /* Don't panic on error */
+                                    __LINE__))
+        {
+          case invalid:
+            return NULL;
+
+          case no_array:
+            return lc_all;
+
+          case only_element_0:
+            SAVEFREEPV(locale_names[0]);
+            return locale_names[0];
+
+          case full_array:
+            /* We need to mortalize the desired component, and free the rest */
+            for (unsigned int i = 0; i < LC_ALL_INDEX_; i++) {
+                if (i == index) {
+                    SAVEFREEPV(locale_names[i]);
+                }
+                else {
+                    Safefree(locale_names[i]);
+                }
+            }
+
+            return locale_names[index];
+        }
     }
 
     /* Here, no usable LC_ALL environment variable.  We have to handle each